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in fhewing the lofs of weight In ice on being heated.
for about a minute, I found it began to lofe weight, on which I immediately took it out, and placed it at a distance from the beam. I alfo immediately plunged a ther mometer in the freezing mixture, and found the temperature 10 degrees; and on putting the ball of the thermometer in the hollow at the bottom of the glass veffel, it fhewed 12 degrees. I left the whole for half an hour, and found the thermometer, applied to the hollow of the glafs, at 32°. Every thing now being at the fame temperature, I weighed the glafs containing the ice, after wiping it carefully, and found it had lot and five divifions; fo that it weighed river water poured into it, and was, all but one divifion, more than hermetically fealed, fo that the when the water was fluid. whole, when perfectly clean, weighed 2150 of a grain exactly; the heat being brought to 32 degrees, by placing it in a cooling mixture of falt and ice till it just began to freeze, and fhaking the whole together.
"The beam I made ufe of was fo adjusted as that, with a weight between four and five ounces in each scale, so part of a grain made a difference of one divifion on the index. It was placed in a room, the heat of which was 37 degrees of Fahrenheit's thermometer, between one and two in the afternoon, and left till the whole apparatus and the brafs weights acquired the fame temperature.
"A glass globe, of three inches diameter nearly, with an indentation at the bottom, and a tube at the top, weighing about 451 grains, had about 1700 grains of New
"After it was weighed it was again put into the freezing mixture, and let stand for about 20 minutes; it was then taken out of the mixture part of the water was found to be frozen; and it was carefully wiped, firft with a dry linen cloth, and afterwards with dry washed leather; and on putting it into the scale it was found to have gained about the part of a grain. This was repeated five times: at each time more of the water was frozen, and more weight gained. In the mean time the heat of the room and apparatus had funk to the freezing point.
"When the whole was frozen, it was carefully wiped and weighed, and found to have gained of a grain and four divifions of the index. Upon standing in the fcale
"I now melted the ice, excepting a very fmall quantity, and let the glafs veffel exposed to the air in the temperature of 32 degrees for a quarter of an hour: the little bit of ice continued nearly the fame. I now weighed it, after carefully wiping the glafs, and found it heavier than the water was at first, one divition of the beam. Laftly, I took out the weights, and found the beam exactly bas lanced as before the experiment.
"The acquifition of weight found on water's being converted into ice, may arife from an increase of the attraction of gravitation of the matter of the water; or from fome fubftance imbibed through the glafs, which is neceflary to render the water folid.
"Which of thefe pofitions is true may be determined by forming a pendulum of water, and another of ice, of the fame length, and in every other refpect fimilar, and making them fwing equal arcs. If they mark equal times, then certainly there is fome matter added
to the water. If the pendulum of ice is quicker in its vibrations, than the attraction of gravitation is increased. For there is no pofition: more certain, than that a fingle particle of inanimate matter is perfectly incapable of putting itself in motion, or bringing itself to reft; and therefore that a certain force applied to any mafs of matter, fo as to give it a certain velocity, will give half the quantity of matter double the velocity, and twice the quantity, half the velocity; and generally a velocity exactly in the inverfe proportion to the quantity of matter. Now, if there be the fame quantity of matter in water as there is in ice, and if the force of gravity in water be part lefs than in ice, and the pendulum of ice fwing feconds, the pendulum of water will lofe of a fecond in each vibration, or one fecond in 28000, which is almoft three feconds a day, a quantity eafily measured.
I fhall just take notice of an opinion which has been adopted by fotne, that there is matter abfolutely light, or which repels instead of attracting other matter. I confefs this appears abfurd to me; but the following experiment would prove or difprove it. Suppofing, for inftance, that heat was a body, and abfolutely light, and that ice gain ed weight by lofing heat; then a pendulum of ice would fwing through the fame arc in lefs time than a fimilar pendulum of water; for the fame power would not only act upon a lefs quantity of matter, but a counter-acting force would also be taken away.
"Till the experiment of the pendulum can be made, or fome other equally certain be fuggefted and made, it would be wafting time to enter into conjecture about the
caufe of the gain of weight in the converfion of water into ice in a glafs veffel hermetically fealed.
"Ifhall only observe, that heat certainly diminishes the attractions of cohesion, chemistry, magnetifm, and electricity; and if it fhould alfo turn out, that it diminishes the attraction of gravitation, I should not hesitate to confider heat as the quality of diminution of attraction, which would in that cafe account for all its effects.
"We come, in the next place, to take notice of the fecond part of the experiment, viz. that the ice gained an eighth part of a grain on being cooled to 12 degrees of Fahrenheit's thermometer. In this cafe, a variation may arife from the contraction of the glafs veffel, and confequent increase of specific gravity in proportion to the air. But it is unneceffary to obferve, that this would be fo very fmall a quantity as not to be obfervable upon a beam adjusted only to the degree of fenfibility with which this experiment was tried. In the fecond place, the air cooled by the ice above the fcale becoming heavier than the furrounding atmosphere, would prefs upon the scale downward with the whole force of the difference. If a little more than half a pint of air was cooled over the fcale to the heat of the ice and glafs containing it, that is, 20 de grees below the freezing point, the difference, according to general Roy's table, would have been the eighth part of a grain, which was the weight acquired; but the air within half an inch of the glafs veffel being only one degree below the freezing point, I cannot conceive, that even an eighth part of a pint of air could be cooled over the icale to 20 degrees below the freezing point; nor that the whole difference
than air, was not unknown to him. Fruits, fays he, viewed through glafs, appear much larger, and the intervals between pillars longer. The ftars, alfo, appear magnified in a humid atmosphere. If a ring be put into a bowl of water, and viewed there, it feems to approach to the eye, or in other words is magnified, which, the fame author observes, is the cafe with every body viewed through a fluid. Seneca fays here exprefly, that water, as a medium, has the fame effect with glass.
"There is a remarkable paffage in Seneca, relative to the effect of glafs cut angularwife, or into a prifmatic form, in feparating the rays of light, when held tranfverfely in the fun's rays. From the expreffion he ufes concerning it, we may think fuch inftruments were not uncommon. Pliny, however, feems to have had the most complete information concerning glafs. He mentions its being of Phoenician origin, like many other great difcoveries. It was firft made of fand, found in the river Belus, or Belcus, a fall iver of Galilee, running from the foot of Mount Carmel, as is teftified by a variety of authors. The invention of it is faid to have been owing to fome merchants, who, coming thither with a hip laden with nitre, or foffil alkali, ufed fome pieces of it to fupport the kettles in which they were dreffing their meat upon the fands. By this means a vitrification of the fand beneath the fire was produced, and thus afforded a hint for this manufacture.
"The Latin writers are more particular. Lucretius was, undoubtedly, acquainted with glafs, and its qualities. In his fourth book, he remarks the difference between founds and the images of objects: the former paffing through any openings, however curved or winding, but the latter, being broken and confufed, if the paffages through which they come are not traight or direct. As an inftance, he adduces glafs, the pores of which he fuppofes to be direct or rectilinear. This, though only true with fome limitations, fhews him to have had no inconfiderable knowledge of the fubjects in question.
"Horace likewife fpeaks of the clearness and brightnefs of glafs, in terms that fhew the ait to have been arrived at a high degree of perfection. Martial mentions glafs in fuch a manner, as fhews it to have been not uncommon in his time for drinking veffels, and alfo of fo clear and tranfparent a texture, as to admit an accurate examination of the liquor contained in them.
Seneca well understood the magnifying powers of glafs, when formed into a convex fhape. A glafs globe, he fays, filled with water, makes letters viewed through it appear larger and brighter. The magnifying power of glafs, confidered as a more denfe, and, of courfe, a more refrangible medium
"Clear pebbles, fhells, and other kinds of foffil fand, were alfo employed, In India, rock cryftal was ufed, and, on that account, the Indian glafs was preferred to any o ther. It was firit melted with the foffil alkali, in proportion of three
of the latter to one of the former (which has continued to be the flux for glafs from the earliest to the prefent times) in furnaces, into maffes of a dull black colour. These were again melted by the refiners, either into a colourlefs glafs, or tinged of any hue they thought proper. The grofs mafs, from the firft fufion, feems to have been called ammonitrum, and probably did not differ much from the lapis obfidianus, which is faid to have been of Ethiopian or Egyptian origin. It is faid to have been a kind of black vitreous fubftance, but ftill pellucid, which was used for cafting into large works. Pliny fays, he faw folid ftatues of the emperor Auguftus made of this material; and the fame emperor dedicated four elephants of the fame fubitance in the Capitol. It appears to have been known from great antiquity, as Tiberius Cæfar, when he governed that country, found a statue of Menelaus of this compofition. Xenocrates likewife, according to Pliny, fpeaks of the fame compofition, as in ufe in India, Italy, and Spain. Sidon in Phoenicia had been, in early times, famous for glafs. In the time of Pliny, that of the Bay of Naples was preferred.
"The Romans were acquainted with the art of engraving upon, or cutting glafs, which is exprefly mentioned by Pliny, and confirmed by the antique gems fo frequently found. It was formed either by blowing it with a pipe, grinding it in a lathe, or catting it in a mould like metal. The colours principally in ufe were an obfcure red glafs, or perhaps rather earthen ware, called hæmatinon; one of various colours, called myrrhinum, a clear red, a white, a blue, and indeed most other colours.
"The perfectly clear glass was, however, moft valued. Nero gave for two cups, of no very extraordinary fize, with two handles to each, upwards of fix thousand feftertia, or above fifty thoufand pounds fterling. But, though the fineft kinds. of glafs were fo valuable and rare, yet I apprehend, from the frequent mention of glafs in Martial, and from what Pliny fays, that glafs for drinking vefiels had nearly fuperfeded the ufe of gold and filver, that the inferior forts must have been common enough.
"Pliny likewife mentions the effects of hollow glafs globes, filled with water, in concentrating the rays of light, fo as to produce flame in any combuflible fubitance upon which the focus fell, and relates, that fome furgeons in his time made ufe of it as a cauftic for ulcers and wounds.
"He was alfo acquainted with the comparative hardnefs of gems and glafs, as he obferves, that the lapis obfidianus would not fcratch gems. And he likewife mentions the counterfeiting of the natural gems by glafs, as a very lucrative art, and in high perfection in his time; and the fame feems to be con firmed by Trebellius Pollio. Vopifcus fays, that Firmus furnished his houfe with fquare pieces of glafs, faftened together with bitumen or other fubfiances; but whether they were to ferve for windows, or as reflectors of the light and objects, does not appear.
"As fpecula, or metal reflectors, in the prefent age, bear fome reference to glafs, and as they were in confiderable ufe among the ancients, I fhall here fubjoin a few words concerning them.
"The antiquity of fpecula, or metal reflectors, muft, according to Plutarch, have been very great. Не
96 to 100 degrees, which happened in the experiments, rather to the acceleration of the blood, than to the influx of heat from the external air. While the caufe of animal heat remains unknown, it would be prefumption to affert, that there are the only means by which the body is enabled to relift the effects of external heat. There may be others; and it is not unreasonable to fuppofe, that as external cold, perhaps by its tonic influence, increases the power of the body to generate heat, to external heat may diminish that power, and thus leffen the quantity of heat generated within, while the evaporation, produced by the fame caufe, guards it againit receiving any acceffion from without.”
AN ESSAY on the ASCENT of VAPOUR, BY ALEXANDER EASON, M. D.
[From the fame Publication. ]
HERE are few phenomena in nature, which have puzzled philofophers more, than the afcent of vapour; and the different theories laid down by doctors Halley and Defaguliers, have been rejected, while another, not lefs liable to objections, has been almost univerfally received.
"This theory, which I fhall presently mention, was at first invented by a French gentleman, Monfieur le Roi, and afterwards revived by Lord Kaimes, and doctor Hugh Hamilton. It is this-that the air diffolves water, as water does faline fubftances: the folution being perfect, the air will become transparent.
performed without air; but Mr. Watt, contrary to the theory fup. ported by Lord Kaimes and Dr. Ha milton, has proved, that when was ter in vacuo was boiled with a degree of heat very little greater than that of the human body, the fteam came over, and was condensed in the refrigeratory. But he relates, that the evaporation was not quicker than in the open air.
"2. Were the doctrine of folution true, the air would be heavier, the more water it contained; and, as clouds contain a great portion of water, they ought to float on the furface of the earth, and not in the higher regions, as we daily observe,
"Objections. 1. Were this the3. We never could expect any ory true, evaporation could not be rain, unleis the air were fuperiatu