I5 2 HISTORY OF CHEMISTRY. [LECTURE IX. 



persons took a part in this important crisis in chemistry, but 

 they were valiant champions who conquered for us this domain. 

 And, once set foot upon, the ground was secure, despite the 

 contradiction of an authority whose words, although they had 

 been observed in other cases in a scrupulously conscientious 

 manner, were now spoken to the winds. 



It is to Graham that the first impulse towards an alteration 

 in the views regarding acids was due. Graham's investigation 

 of phosphoric acid, and the way in which he states his results 

 the former free from preconceived notions and hypotheses, 

 and the latter clear and definite show us that we have to do 

 with an acute and clear-minded thinker. When regard is paid 

 to the ideas that were necessarily introduced as the direct 

 result of the investigation, and when those intellectual advances 

 are considered which were occasioned not exclusively, it is 

 true, but still to a great extent by Graham's labours, it must 

 be conceded that so much has seldom been accomplished by a 

 single investigation. 



As a consequence of Clark's investigation of phosphoric 

 acid, 3 the view had been arrived at that this acid existed in 

 two isomeric conditions, which, in their salts in particular, were 

 stated to present great differences. 4 Common sodium phos- 

 phate gave a yellow precipitate with neutral silver salts, and 

 the liquid possessed an acid reaction ; the pyrophosphate, on 

 the other hand, precipitated white silver pyrophosphate and 

 the neutrality remained. It was known, of course, that the 

 one sodium salt crystallised with more water than the other, 

 but this was regarded as water of crystallisation, and no im- 

 portance was attached to it ; so that the two acids were looked 

 upon as isomeric modifications. 5 Graham rectified this mistake. 

 He succeeded in throwing light upon this hitherto obscure 

 subject by proving that the water which was contained in the 

 hydrated acids should not be disregarded as inessential to their 



3 Edinburgh Journal of Science. 7, 298 ; Schweigger's Journal. 57, 421. 

 4 See also Stromeyer, Schweigger's Journal. 58, 123. 5 Compare Ber- 

 zelius, Lehrbuch. Third Edition, 2, 60. 



