Prof. Liebig o« the Constitution of the Urine in Man, S^c. 453 



obscura; and where, from situation or otherwise, there is a 

 difficulty in observing the colour of the plate during the pro- 

 cess of iodizing, it may be removed from the iodine vessel and 

 its colour examined by the direct light of the sun without risk 

 of injury; for when returned to the iodine or bromine vessel 

 for a moment the effect of light is wliolly destroyed. 



Perhaps the most valuable practical application of these 

 facts is in the use of the same plate for receiving several im- 

 pressions. When, on taking a portrait or the picture of any 

 object liable to move, there is reason to suppose that the mo- 

 lion of the person or object has rendered the operation use- 

 less, it is not necessary to throw aside the plate on which the 

 imperfect impression has been taken, and resort to the tedious 

 process of cleaning and preparing another ; it is only necessary 

 to treat the plate in the manner already pointed out, and it is 

 again equal in every respect to a newly prepared plate ; and 

 this treatment may be repeated until by the slow accumulation 

 of too thick a film of iodide of silver, the plate no longer pos- 

 sesses the same degree of sensitiveness to light. 

 Temple Row West, Birmingham, GeoRGE ShaW. 



November 15, 1844. 



LXXVI. On the Constitution of the Urine in Man and Carni- 

 vorous Animals. By Justus Liebig, M.D.^ Ph.D., F.R.S., 

 M.R.I. A., Professor of Chemistry in the University of 

 GiesseW^. 



T T is a very curious and remarkable fact that all the analyses 

 -*- and examinations hitherto made of the urine of man and 

 of carnivorous animals have not yet afforded us any satisfac- 

 tory answer to the question. What is the substance which im- 

 parts to that fluid its property of reddening blue vegetable 

 colours ? 



In most physiological and chemical works we find the acid 

 reaction of urine ascribed either to uric acid or to lactic acid ; 

 but no positive and definite proof is given of the presence of 

 the latter acid in urine. 



The acidification of milk, that is, the formation of lactic 

 acid, is dependent upon the milk-sugar contained in milk ; 

 this substance being in contact with caseine in a state of de- 

 composition and transformation, undergoes, by its means, an 

 alteration, which consists in its elements transposing and ar- 

 ranging themselves into lactic acid without the addition or se- 

 paration ofanj' one atom. Crystallized milk-sugar (C,2Hi20i2) 

 and hydrate of lactic acid (Cg Hg Og) have one and the same 



* From tlie Lancet, Nos. xi. and xii., by the obliging permission of T. 

 Wakley, Esq., M.P. 



