46 HISTORY OF CHEMISTRY. [LECTURE III. 



sible, whilst the latter assumed that substances combine in a 

 few definite proportions only. We call a substance a com- 

 pound when it contains its constituents in invariable pro- 

 portions. 



I do not know whether the difference between the two con- 

 ceptions has been made clear. In order to appreciate the full 

 bearing of the question, a person must himself have required 

 to decide as to whether he had a mixture or a compound in 

 his hands. Even yet we are without a definition which shall 

 suffice for every case ; that is, without such a definition as 

 Berthollet repeatedly demanded from Proust. It is true that 

 we have certain means of judging with respect to chemical 

 compounds, as, for instance, capacity for crystallising, and in- 

 variable melting point in the case of solids, and constant 

 boiling point in the case of liquids. Yet these are frequently 

 insufficient. I need only recall the phenomena of isomorphism, 

 when we must admit that mixtures also can crystallise. I men- 

 tion the solutions of hydrochloric acid, hydriodic acid, etc., in 

 water, regarding which Roscoe has proved that they are only 

 mixtures (solutions), and we must admit that these likewise 

 can possess a constant boiling point. In short, this distinction 

 forms one of the most difficult and most important problems, 

 and in point of fact it is often insufficiently attended to. In the 

 study of chemical papers opportunity is often afforded of observ- 

 ing how errors have arisen through neglect of this very matter, 

 How often have formulae been advanced for substances and 

 theoretical conclusions based upon their existence, before their 

 compound character has been conclusively settled ! The pur- 

 pose of the foregoing remarks is to serve as a warning against 

 any such error, and I therefore hope to be excused for having, 

 for a short time, quitted my proper theme. 



