60 THE PRINCIPLES OP SCIENCE. 



Such experiments will in many subjects only give the 

 first hint of the nature of the law in question, but will 

 not admit of any exact measurements. The parabolic form 

 of a jet of water may well have suggested to Galileo his 

 views concerning the path of a projectile ; but it would 

 not serve now for the exact investigation of the laws 

 of gravity. It is not likely too that capillary attraction 

 could be exactly measured by the use of inclined plates 

 of glass, and the tubes would probably be better for 

 precise investigation. As a general rule, these collective 

 experiments would be most useful for popular instruction 

 and illustration of the laws of science. But when the 

 curves and figures produced are of a precise and per- 

 manent character, as in the coloured figures produced by 

 crystalline plates, they may admit of exact measurement, 

 and may often be the only mode of approaching the ques- 

 tion. Newton's rings, diffraction fringes, and other effects 

 of the interference of light, allow of very accurate 

 measurements. 



Under the class of collective experiments we may per- 

 haps place those in which we render visible the motions 

 of a mass of gas or liquid by diffusing some opaque 

 substance in it. The behaviour of a body of air may 

 often be studied in a beautiful way by the use of smoke, 

 as in the production of smoke rings and jets. In the case 

 of liquids lycopodium powder is sometimes employed. To 

 detect the mixture of currents or strata of liquid, I em- 

 ployed exceedingly dilute solutions of common salt and 

 silver nitrate, which produce a very visible cloud wherever 

 they come into contact . Atmospheric clouds often reveal 

 to us the movements of great volumes of air which would 

 otherwise be quite unapparent. 



'Philosophical Magazine,' July, 1857, 4th Series, vol. xiv. p. 24. 



