Dr. Roget in Reply to Mr. Hume on his Test for Arsenic. 179 



tina, the usual crystalline appearance of potash- Uiuriat of 

 platina took place immediately; but by slow spontaneous 

 evaporation, other and more abundant needle shaped cry- 

 stals ot" soda-mnriat of platina made their appearance. 



** My conclusion, therefore, (which I hope will now also 

 be yours,) is precisely as bef<ire; namely, that the poiash 

 which exists in the animal fluids is in the state of mniiat, 

 and that the whole of the uncombined alkali is soda; and 

 as it is a known fact that muriat of potash is in some de- 

 gree soluble in alcohol, the circumstance which led you 

 into error is easily explained. 



*' I have only further to add, that the fact which T have 

 endeavoured to establish by a specific incjuiiy, ought to 

 have been interred from principle; for it is well known 

 that potash has a stronger attraction for the muriatic acid 

 than si-da; and indeed I understand tliat it is a connnon 

 process in some manufaciurts, to obtain soda by the action 

 of potash-ley on n)uriat of soda. 



" Believe me ever, &c. &c. 



'' Alex. Marcet. 



**■ P.S. — Since the above was written, I have, in conse- 

 quence of vour suggestion, ' that ttic blood of graminivurous 

 animals might periiaps yield potash instead of soda, on 

 account of iheir living e.xclu--ively upon vegetable tood/ 

 examined bullock's blood with a view to ascertain this cir- 

 cumstance; and as there was no difficulty in procuring any 

 quantity of that blood, I had some gallons evaporated, troni 

 which 1 jirocured some ounces of salts, in o;der to satisfy 

 those who ihink that nothing certain can be inferred honn 

 experiments u))on a small scale. However, the results 

 were precisely sirnilar, except that the crystals of sulphat 

 and nil rat oi soda obtained by the processes above detailed, 

 were of much larger dimensions than in any of my former 

 cxperinu-nts." 



XXXlll. Vr. Roget in Reply to Mr. Hume on his Test 

 for Arsenic. 



To Mr. Tilloch. 



Sir, In the last number of your Magazine, Mr. Hume 

 has thought proper to repeat a complaint which he had 

 made in the Medical and Physical Journal for August last, 

 of my not having paid sufficient attention, in the paper to 

 which he alludes *, to a method which he had proposed 



• " A Case of Recovery from the FfJects of Arsenic," published in the 

 Kcond volume ui the AleUico-Chiriir^H al 'i'ransactiout. 



M 2 some 



