301 
History of Chemical Discoveries, 
less rapidly than the nitro-^saccharate of lime. With magnesia 
it also gives a salt in small crystalline grains, which does not 
attract humidity like the nitro-saccharate of magnesia, 
4. That other uncrystallisable and sapid substances, ana- 
logous to certain vegetable principles, are produced by the re- 
action of the sulphuric acid on the most insoluble animal sub- 
stances.— Abridged from the Ann, de Chim. tom. xiii. p, 113. 
II. On the Caloric of a Vacuum. By M. Gay Lussac. 
In order to prove by an experiment on a large scale, that a 
thermometer placed in vacuo indicates no variation of tempera- 
ture when the vacuum is suddenly diminished or enlarged, M. 
Gay Lussac employed a barometer 75 millimeters in diameter, 
and nearly a metre long. It was a tube of glass shut up at its 
upper extremity by a plate of metal perforated with two holes, 
one for admitting an air thermometer into the barometric va- 
cuum, and the other for establishing, by means of a very nar- 
row tube, with a stop-cock, a communication between the large 
tube and a good air-pump, for making the vacuum. This 
tube, when filled with mercury, had a considerable weight, and 
was fixed between two pillars by means of a large collar carry- 
ing two gudgeons, upon which it was supported. The cistern in 
which its lower end was plunged, was about five decimetres deep. 
It was suspended by two cords, coiled round a barrel with a 
winch, and by raising or depressing it, the extent of the baro- 
metric vacuum could be varied at pleasure. The ball of the air 
thermometer was placed in the upper part of the tube, near the 
metallic plate already mentioned. It was nearly of the same 
diameter as the interior of the tube ; and the stem of the ther- 
mometer, in which there was a small column of coloured water, 
was, on the contrary, of a very small diameter. The sensibili- 
ty of this instrument was very great, and the calculation of its 
dimensions showed, that the six hundredth part of. a degree oc- 
cupied nearly a millimetre upon the scale, and was therefore 
very appreciable. 
Having made as perfect a vacuum as possible, the cistern 
was raised till the column of mercury reached the top of the 
tube, and even rose very near the stop-cock. Upon shutting 
the stop-cock, and making the cistern descend, a vacuum is ob- 
VOL. III. NO. VI, OCTOBEE 1820. 
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