PROPERTIES OF THE BLOOD. 70 



But the degree of heat necessary for their coagulation is 

 not the only circumstance in which the lymph differs from the 

 mucilage of the serum ; they differ more remarkably in the 

 former coagulating when exposed to air, whilst the serum un- 

 dergoes no such change. 



When the serum is coagulated by heat, a watery fluid can 

 be pressed out of it ; and this fluid the learned M. de Senac 

 distinguishes by the name of serosity (LIV). 



This serosity contains the neutral salts of the blood, and 

 although it has been exposed to the heat of boiling water, yet 

 it still contains a part of the mucilaginous substance, which is 

 combined with the water in such a manner as not to be coagu- 

 lated by the heat of boiling water, till a part of the water is 

 evaporated by boiling ; and then it coagulates, and appears 

 not very much unlike the mucus spit up from the aspera arteria 

 in a morning. 



When the mucilaginous part of the serum has been coagu- 

 lated by heat, it cannot again ' be dissolved in the serosity, 

 except by putrefaction or by the addition of some chemical 

 substance, and then it differs from what it was before, particu- 

 larly in its not being coagulable by heat. 



But if the serum be exposed to a less degree of heat than is 

 required for its coagulation, for example, to that of 100, it is 

 gradually inspissated into a brownish solid mass, and this mass 

 can readily be dissolved again merely by the addition of water ; 



(LIV.) Mr. Hunter a claimed the discovery of the serosity. But it 

 was probably known before 1760 ; for in that year Dr. Butt distinctly 

 mentioned the serosity, and referred inaccurately concerning it, like 

 Mr. Hewson afterwards, to Senac, as I have more fully explained in the 

 Introduction. Dr. Cullen b described the properties of the serosity 

 more perfectly. Mr. Hunter subsequently made many experiments on 

 it; he believed, but .by no means proved, that it is identical with the 

 gravy of cooked meat, and observed that more gravy is contained in the 

 meat of old than of young animals. The serosity is probably only the 

 serum wanting the albumen which has been coagulated by heat ; so 

 that, caeteris paribus, the weaker the serum in albumen the more serosity 

 there would be. 



Dr. Bostock c has given the best account of the serosity, and full re- 

 ferences to the observations of others on the subject. 



a On the Blood, pp. 17, 32-4. 4to, c An Elementary System of Physiology, 

 Lond. 1794. i, 473, 2d ed. 8vo, Lond. 1828. 



b Institutions of Medicine, 252, 2d ed. 

 8vo, Edin. 1777. 



