VOL. XXVI.]| PHILOSOPHICAL TRANSACTIONS. 49I 



becomes more than twice as light as common air, and must therefore rise so 

 much the faster. Besides this, the air itself being rarefied must necessarily 

 rise up, and the force of its motion carry with it what bodies it meets 

 with in its way; which will more easily ascend according as their surfaces are 

 greater in proportion to their weight. And the author asserts, that if the spe- 

 cific weight of bodies, the force that impels them, and the measure of their 

 surfaces be rightly considered, it will not be difficult to give an account of vola- 

 tility and fixedness, and all the appearances of distillation and sublimation ; in 

 which last only solid bodies are raised by the force of fire. 



In his lecture on fermentation, the author deduces the cause of ebullition 

 and effervescence from the attractive force of the particles of matter, and par- 

 ticularly those of salts; which he says are very simple and small bodies, and in 

 proportion to their bulk very solid, and must of consequence be endowed with 

 a very strong attractive power. On which account, and that of the smallness 

 of their force of cohesion, he shows the reason why they are so easily dissolved 

 into water, and not in spirit of wine; as also why water can only dissolve a 

 certain proportion of these salts, so that whatever quantity greater than this is 

 immersed in water, remains undissolved. 



The solution of all other bodies is to be deduced from the same principles ; 

 but to understand them rightly, it is necessary to estimate the wideness of the 

 pores of these bodies, the force by which the parts cohere together, and the 

 efficacy or force of motion in the parts of the menstruum ; which last arises 

 from the difference of attraction of the particles of the menstruum to one 

 another, and to the parts of the body, and from their elasticity. And upon 

 these grounds he explains the various phasnomena of solutions ; particularly of 

 that hitherto unaccountable one of aqua regia dissolving gold, but not silver, 

 whereas aquafortis, of which the aqua regia is made, dissolves silver, but not 

 gold ; which he illustrates and reduces to a plain calculation. 



In the lectures on digestion and extraction, he shows that there is a tenacity 

 in all fluids, by which their parts do in some measure cohere together, and 

 hinders their effects from being the same as in a perfect fluid. He gives the 

 method of estimating this tenacity, and of finding out the proportion it may 

 have to the weight of other bodies ; and from thence he explains how small 

 particles of matter, that are either specifically lighter or heavier than the fluid 

 may be sustained in it, which he explains by a calculation ; and shows, that if 

 the gravity of the body be to the tenacity of the fluid, as p to 1, if the body 

 be divided into parts, whose diameters are to the diameter of the whole as 1 

 to p, then these bodies may be sustained in the fluid, though specifically lighter 

 than themselves. , , ,^ 



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