80 PROPERTIES OF 



and the serum seems to possess the same properties that it did 

 before, particularly it is capable of being coagulated by heat. 

 But care must be taken not to add more water than it had 

 lost, for if more be added than was evaporated, it alters its 

 properties of coagulation. 



In this circumstance of being inspissated, and again ren- 

 dered dissolvable in water, the serum agrees with the white of 

 an egg, 1 but differs from the coagulable lymph, which even 

 when mixed with a neutral salt (viz. true Glauber's salt), can- 

 not be inspissated and dissolved again without coagulation. 



If fresh serum be diluted with an equal quantity of water, 

 and then exposed to heat, it does not coagulate in that of 

 160, as when undiluted (see Note xvu), nor even in the heat 

 of boiling water, as I have lately observed, but it can now 

 be boiled without immediately coagulating. And as the water 

 evaporates, a pellicle is formed on the surface, which be- 

 comes thicker and thicker as the evaporation advances. This 

 pellicle seems to be the mucilage coagulated, for it cannot 

 again be dissolved in water like the inspissated serum. 



Milk, when boiled, has its mucilage or coaguable part sepa- 

 rated in like manner, in the form of a pellicle, in proportion 

 as the evaporation takes place. And both milk and serum, 

 whether diluted, or not, agree in being coagulated by rennet 

 when exposed to heat (LV). So that milk seems to be made 



1 See Newman's Chemistry, sect. ix. 



(LV.) Dr. George Fordyce a says that the serum may be 'coagulated by 

 a juice secreted in the stomach. Dr. Davy b found that serum is not 

 coagulated by calf s rennet ; and this observation is confirmed by the 

 following note with which he has favoured me : " I have repeated the 

 experiment with rennet, and with the same negative result, whether 

 using the weak brine in which the stomach of the calf was infused, or 

 a portion of the membrane itself ; whilst both coagulated milk, they 

 had no effect on serum, not even appearing to render it coagulable at 

 a lower temperature. The serum used was from the blood of the sheep. 

 If Hewson were deceived, as I am inclined to believe he was, it may 

 have been from his employing too high a temperature, and attributing 

 to rennet what was the effect of heat.' 1 I have tried calf s rennet on 

 the serum of the horse, without producing any coagulation, at a tem- 

 perature gradually raised from 65- 103. 



a Elements of the Practice of Physic, b Researches, Physiol. and Anat. ii, 97. 

 part i, p. 3, 8vo, Lond. 1770. 



