i7i Fluids. 269 



of the motions into which the air is thrown on those oc- 

 casions, notwithstanding that they are extremely compli- 

 cated, and would be very difficult to describe. 



It was by the motions of the very fine particles of dust, 

 which by accident had been mixed with the spirits of 

 wine in my large thermometer, and which, when strongly 

 illuminated by the direct beams of the sun, became visi- 

 ble, that I first discovered the internal motions in that 

 Fluid which take place when it is cooling ; and, availing 

 myself of this kind hint, I contrived to render the in- 

 ternal motions of water equally visible. This I imme- 

 diately saw could be done with the utmost facility if I 

 could but find any solid body of the same specific grav- 

 ity as water, which would be proper to mix with it, — 

 that is to say, that would not be liable to be dissolved 

 by it, or to be reduced to such small particles as to be- 

 come itself invisible ; but such a substance was not to 

 be found. On reflection it occurred to me that it is very 

 fortunate that such substances do not abound, for other- 

 wise we should find great difficulty in procuring water in 

 a pure state. 



Not being able to find any solid substance fit for my 

 purpose, of the same specific gravity as pure water, I 

 was obliged to have recourse to the following strata- 

 gem. 



Looking over the tables of specific gravities, I found 

 that the specific gravity of transparent yellow amber was 

 but a little greater than that of water, being 1.078, while 

 that of water is i.ooo ; and it occurred to me, that, by 

 dissolving a certain quantity of pure alkaline salt, I 

 might augment its specific gravity, or rather bring the 

 specific gravity of the solution to be precisely equal to 

 that of the amber, without impairing the transparency 



