222 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777- 



used in thcformer experiments: these suspicions induced him to try the following 

 experiments. 



A piece of leather dressed in alum, known by the name of white sheep-skin, 

 of about 4 inches diameter, which had been soaked in oil and tallow about a 

 year before (such as was used to place the receiver on in the 1st and 2d experi- 

 ments) was put into the receiver ; the pump was tlien worked, and the barometer 

 gage indicated a degree of exhaustion of nearly 300 ; but on tlie admission of 

 the air the pear-gage indicated a degree of exhaustion of 4000. But the piece 

 of leather being taken out, the pump was then worked, and the degree of ex- 

 haustion appeared by both the barometer and pear-gages to be about 600, as 

 before. ! 



Again, a cylinder made of a piece of box-wood, 1 inch in diameter and 3 

 inches in length, was put into the receiver ; the pump was then worked, and the 

 degree of exhaustion appeared by the barometer-gage to be 300, but by the pear- 

 gage 1 6,000. These experiments have often been repeated, but the result was 

 seldom the same. When leather soaked in oil and tallow has been put into the 

 receiver, the pear-gage has sometimes indicated a degree of exhaustion of 

 20,000, and sometimes no more than 500; it likewise differs very much from 

 the box-wood, which may perhaps be owing to different degrees of heat and 

 moisture. 



From these experiments it is evident, that there arises an elastic vapour from 

 theleather dressed in alum and soaked in oil and tallow, and also from the piece 

 of box-wood, when the weight of the atmosphere has been partly taken off 

 by the action of the pump ; and that this vapour presses on the surface of the 

 quicksilver in the tube of the long barometer-gage, and of that in the cistern 

 of the short one ; and that consequently the testimony of both these gages must 

 be influenced by this vapour, as well as by the small remainder of common air : 

 but as it is the nature ot the pear-gage not to give its testimony till the remain- 

 ing air contained in it is pressed, so as to become of the same density of the at- 

 mospliere ; and as this vapour cannot subsist in the form of vapour under that 

 pressure, this gage is ncjt at all influenced by it, but indicates the remaining 

 quantity of permanent air only. ' • 



Seeing thus what a considerable quantity of vapour arose from the compound 

 of leather, alum, oil, and tallow, Mr. N.'s next object was to find out from 

 which of those substances it chiefly arose ; how far he succeeded appeared by 

 other experiments. From these it appeared that the elastic vapour, which caused 

 so great a difference in the testimony of the gages, arose principally from the 

 leather, and but little from the tallow, oil, or alum : it even appears by one ex- 

 periment, that it came from the leather, and supplied the place of the exhausted 



