NATURE OE THE HUMOURS OF THE EYE. 2S 



©X to be 30 grains ; and its fpecific gravity was, as I before 

 dated, 10765. I then pared away all the external part, in 

 every direction, till there remained but fix grains of the centre, 

 and the fpecific gravity of ihefe fix grains, I found to be 1 1940. 

 From this it would feem, that the denfity increafes gradually, 

 from the circumference to the centre. 



It is not furprifing that the cryftalline humour fiiould beltis very fubjedt 

 fubje6l to diforders, it being wholly compofcd of animal matter ^^ ' ?^ ^" 

 of the moft perifhable kind. Fourcroy fays, that it is fome- table nature. 

 times found ofleous in advanced age. Albumen is coagulated 

 by many methods ; and, if we fuppofe that the fame changes 

 can take place in the living eye as in the dead animal matter 

 of the chemifts, it will be eafy to account for the formation of 

 the catara6t ; a diforder which cannot be cured but by the 

 removal of the opaque lens. If a fufficient number of obferva- 

 tions were made refpeding the frequency of the cataract in 

 gouty habits, fome important conclufions might be drawn, as 

 to the influence of phofphoric acid. In caufing the diforder, 

 by the common efl^ecl of acids, in coagulating albumen. 



IV. 



A Letter from Mr. Irvine concerning the late Dr. Irvine, of 

 Glafgow, his Dottrine, zvMch ufcribes the Dijiippearancc of 

 Heat, without Increafe of Temperature, to a change of Capacity 

 in Bodies, and that of Dr. Black, tihich fuppofcs Caloric to 

 become latent by Chemical Combination ivitli Bodies ; ivith par- 

 ticular Remarks on the Mijlakes of Dr. Thompson, in his 

 Accounts of thefe Do6trines. 



To Mr. NICHOLSON, 

 SIR, 



In the article Chemiftry in the Supplement to the Encycio- Account given 

 pcEdia Britannica, in moft refpeds excellently written, I could'^^[^^^^ o"the'* 

 not fail to be ftruck with the account there given of the theory inveftigation of 

 of heat, and the mode of inveftigating the natural zero adopted ^^^ "™ °^ ^^^^h 

 by the late Dr. Irvine. Had Dr. Thompfon been indifputably 

 accurate in his opinions, had a mathematically clofe argument 

 left no door for the entrance of doubt, he would fcarcely even 



then 



