208 Dr. Playfair on Transformations 



now seize oxygen, which it could not do from the nitric acid 

 when in a less tense state. The quartation of gold is ob- 

 viously a phaenomenon of the same kind. In these instances 

 the interposing silver much reduces the cohesive or aggregative 

 force of the platinum or gold, which opposes so strongly the 

 action of nitric acid upon them. But when we have every 

 atom of platinum or of gold separated by one of silver, great 

 facility is given to the nitric acid to act upon these metals, 

 especially when the silver at the same time aids them by its 

 assistant affinity. 



We have seen, in the consideration of the previous in- 

 stances of catalysis, that the play of affinities was occasionally 

 so nearly balanced, that a second disturbing cause determined 

 the direction of the action. In the case of non-accendi- 

 ble phosphuretted hydrogen, the addition of another oxi- 

 dizable body, NO4, decided the union of oxygen with the gas. 

 In accendible phosphuretted hydrogen the compound PHg 

 played the same part. When the accessory agent is present 

 in small quantity, the preponderating affinity of the body 

 acted upon shows itself in the result. But, as the ac- 

 tion is due to two affinities nearly equal in amount, it is easy 

 to conceive that the increased quantity of the accessory agent 

 may exactly balance affinities, and that the catalytic phaeno- 

 menon will be prevented. Thus one-twentieth of the volume 

 of binoxide of nitrogen, according to Graham*, added to ac- 

 cendible phosphuretted hydrogen, does not deprive it of in- 

 flammability, the bubbles of gas escaping into the air with a 

 kind of explosion, although one-tenth volume of the same 

 gas altogether prevents the accendibility. This nitric oxide, 

 when pure, does not, like protoxide of nitrogen, render phos- 

 phuretted hydrogen spontaneously inflammable, the reason 

 obviously being that its own affinity for oxygen is more 

 powerful than that of the phosphuretted hydrogen. When 

 added however in such small proportion to the accendible 

 gas that the foreign constituent in it preponderates, then it 

 becomes an accessory to the oxidation, though an increase of 

 the quantity renders it more powerful, and prevents accendi- 

 bility by itself seizing oxygen. Thus also larger volumes of 

 gas, having an affinity for oxygen, but incapable like NO4 of 

 gratifying that desire under ordinary circumstances, may 

 exactly balance the feeble affinity of the foreign accessory 

 body and prevent oxidation. Five volumes of hydrogen, 

 2 volumes of carbonic acid, 1 volume of olefiant gas, and 

 1 volume sulphuretted hydrogen, deprive 1 volume of phos- 



* Phil. Mag., Tliird Series, vol. v. p. 405. 



