304 PHILOSOPHICAL TRANSACTIONS. [aNNO 17 66. 



be 5490 times lighter than water, or near 7 times lighter than common air. 

 But if the density of common air was 850 times less than that of water, then 

 would inflammable air be 9200 times lighter than water, or 10-^ lighter than 

 common air. This method of finding the density of factitious air is very con- 

 venient and sufficiently accurate, where the density of the air to be tried is not 

 much less than that of common air, but cannot be much depended on in the 

 present case, both on account of the uncertainty in the density of common air, 

 and because we cannot be certain but what some common air might be mixed 

 with the inflammable air in the bladder, notwithstanding the precautions used to 

 prevent it ; both which causes may produce a considerable error, where the den- 

 sity of the air to be tried is many times less than that of common air. For this 

 reason, I made the following experiments. 



I endeavoured to find the weight of the air discharged from a given quantity 

 of ?inc by solution in the vitriolic acid, in the manner represented in fig. 16. 

 A is a bottle filled nearly full with oil of vitriol diluted with about 6 times its weight 

 of water; b is a glass tube fitted into its mouth, and secured with lute; c is a 

 glass cylinder fastened on the end of the tube, and secured also with lute. The 

 cylinder has a small hole at its upper end to let the inflammable air escape, and 

 is filled with dry pearl-ashes in coarse powder. The whole apparatus, together 

 with the zinc, which was intended to be put in, and the lute which was to be 

 used in securing the tube to the neck of the bottle, were first weighed carefully; 

 its weight was UQSO grains. The zinc was then put in, and the tube put in 

 its place. By this means, the inflammable air was made to pass through the dry 

 pearl-ashes; by which it must have been pretty effectually deprived of any acid 

 or watery vapours that could have ascended along with it. The use of the glass 

 tube B was to collect the minute jets of liquor, that were thrown up by the ef- 

 fervescence, and to prevent their touching the pearl-ashes; for which reason, a 



air exhausted: and therefore the air will appear lighter than it really is in the proportion of more 

 than 15 to 14, i. e. more than 857 times lighter than water: whereas, if the ball had been weighed 

 in air in both circumstances, the error arising from the compression would have been very trifling. 



It appears, from some experiments that have been made by weighing a ball in air, while ex- 

 hausted, and also after the air was let in, that air, when the thermometer is at 50°, and the baro- 

 meter at 29|, is about SOO times lighter than water. Though the weight of the air exhausted wa« 

 little more than 50 grains, no error could well arise near sufficient to make it agree with Hauksbee'g 

 experiment. Air seems to expand about -^^ part by 1" of heat, whence its density in any other 

 state of the atmosphere is easily determined. The density here assumed agrees very well with the 

 rule given by the gentlemen who measured the length of a degree in Peru, for finding the height of 

 mountains barometrically, and whic his given in the Connoissance des Mouvemens Celestes, annee 

 1762. To make that rule agree accurately with observation, the density of air, whose heat is the 

 game as thai of the places where these observations were made, and which I imagine we may esti- 

 mate at about 45°, should be 798 times less than that of vrater, when the barometer stands at 2y^. 

 — Orig. 



