GELATINOUS SUBSTANCES. 21 



alkalies do Dot change it; it is one of those fatty substances 

 which are not saponifiable. 



The azotized or nitrogenous principles in the human body 

 include what may be called the proper gelatinous and albu- 

 minous substances, besides others of less definite rank and 

 composition, as pepsin and ptyalin, horny matter or keratin, 

 many coloring and extractive matters, &c. 



The gelatinous substances are contained in several of the 

 tissues, especially those which serve a passive mechanical 

 office in the economy ; as the cellular, or fibro-cellular tissue 

 in all parts of the body, the tendons, ligaments, and other 

 fibrous tissues, the cartilages and bones, the skin and serous 

 membranes. These, when boiled in water, yield a material, 

 the solution of which remains liquid while it is hot, but be- 

 comes solid and jelly-like on cooling. 



Two varieties of these substances are described, gelatin and 

 chondrin, the latter being derived from cartilages, the former 

 from all the other tissues enumerated above, and in its purest 

 state, from isinglass, which is the swimming-bladder of the 

 sturgeon, and which, with the exception of about 7 per cent, 

 of its weight, is wholly reducible into gelatin. The most char- 

 acteristic property of gelatin is that already mentioned, of its 

 solution being liquid when warm, and solidifying or setting 

 when it cools. The temperature at which it becomes solid, the 

 proportion of gelatin which must be in solution, and the firm- 

 ness of the jelly when formed, are various, according to the 

 source, the quantity, and the quality of the gelatin; but, as a 

 general rule, one part of dry gelatin dissolved in 100 of water, 

 will become solid when cooled to 60. The solidified jelly 

 may be again made liquid by heating it, and the transitions 

 from the solid to the liquid state by the alternate abstraction 

 and addition of heat, may be repeated several times ; but at 

 length the gelatin is so far altered, and, apparently, oxidized 

 by the process, that it no longer becomes solid on cooling. 

 Gelatin in solutions too weak to solidify when cold, is dis- 

 tinguished by being precipitable with alcohol, ether, tannic 

 acid, and bichloride of mercury, and not precipitable with the 

 ferrocyanide of potassium. The most delicate and striking of 

 these tests is the tannic acid, which is conveniently supplied 

 in an infusion of oak-bark or gall-nuts; it will detect one part 

 of gelatin in 5000 of water ; and if the solution of gelatin be 

 strong it forms a singularly dense and heavy precipitate, which 

 has been named tanno-gelatin, and is completely insoluble in 

 water. 



Chondrin, the kind of gelatin obtained from cartilages, 

 agrees with gelatin in most of its characters, but its solution 



