THE DISCOVERER MUST KNOW HOW TO COMPARE. 329 



all its salts by the same precipitants ; coloured salts are 

 also most commonly derived from the same elements. 

 Many other circumstances besides chemical union disguise 

 essential resemblance and difference. 



In order to be able to detect essential resemblances 

 and differences in substances and actions, we must know 

 practically how to compare them, and this often requires 

 great knowledge of detail, and special experience in mani- 

 pulation, such as would be acquired whilst learning how 

 to experiment and observe. Essentially different sub- 

 stances, or mixtures or compounds of them, are usually 

 distinguished by means of chemical analysis or spectro- 

 scopic observation ; and chemical analysis alone is a very 

 complex and extensive art, and its successful practice re- 

 quires considerable skill and experience. 1 Some substances 

 are apparently so very similar that it is difficult to distin- 

 guish them, and still more so to separate them ; and each 

 different substance is detected in a different way. The older 

 chemists could not distinguish potash from soda ; some- 

 what later, baryta and strontia were confounded together, 

 until Klaproth and Haiiy showed differences of properties ; 

 and until within the last few years potash and caesia could 

 not be distinguished. It is still difficult to accurately 

 separate nickel and cobalt, phosphorus and vanadium, and 

 several of the rare earths. Potash is usually detected by 

 its giving a yellow precipitate with chloride of platinum ; 

 soda by its power of giving yellow light, lithium by its red 

 ray, thallium by its green one, and so on. Each different 

 force, also, is recognised by a different method heat by 

 means of thermometers and thermo-electric piles ; static 

 electricity by means of electroscopes ; current electricity 

 by galvanometers, and so with the rest. By comparing 



1 See Fresenius's Qualitative and Quantitative Chemical Analysis. 



