406 



ORGANS OF GENERATION. 



rendered at first turbid, and when excess is 

 added a white adhesive compound falls: nitrate 

 and per-nitrate of mercury and chloride of tin 

 occasion nearly similar changes. But the me- 

 tallic salt which is the most decided precipitant 

 of gelatin, and which does not affect albumen, 

 is sulphate of platinum ; it throws it down 

 even from very dilute solutions, in the form 

 of brown flocculi, which, when collected and 

 dried, become black and brittle, and which, 

 according to Mr. Edmund Davy, to whom we 

 owe this effective test, consist of about 76 per 

 cent, of sulphate of platinum and 24 per cent, 

 of gelatin and water. 



We now come to the most important and 

 characteristic property of gelatin, which is, 

 that of combining with tannin, and upon 

 which the art of tanning, or the conversion 

 of skin into leather, essentially depends, for 

 the true skin (cutis) of animals consists of 

 a condensed and fibrous form of organised 

 gelatin, and, when properly prepared and im- 

 mersed in a solution of vegetable astringent 

 matter or tannin, it becomes gradually pene- 

 trated by and combined with it, and when 

 dried is rendered insoluble and durable. The 

 tannin of the gall-nut is perhaps that which 

 forms the most insoluble precipitate in gelati- 

 nous solutions, and is therefore the most de- 

 licate test of the presence of gelatin ; but, as 

 albumen is also thrown down by it, the absence 

 of the latter must have been previously ascer- 

 tained. (See ALBUMEN.) A strong infusion 

 of galls occasions a precipitate in water holding 

 less than a five-thousandth part of gelatin in 

 solution, and, if added to a strong solution 

 of gelatin, it throws it down in the form of 

 a curdy precipitate, more or less dense and 

 coloured according to the greater or less excess 

 of the precipitant. The precipitated compound 

 is insoluble in water, dilute acids, and alcohol, 

 and when dried becomes hard and brittle, but 

 again softens and acquires its former appear- 

 ance when soaked in water : it may be termed 

 tanno-gelatin. When tannin is added to a 

 solution of gelatin, the latter being in excess, 

 and especially if it be warm, no precipitate 

 is immediately formed, for tanno-gelatin, when 

 recently precipitated, is to a certain extent 

 soluble in liquid gelatin. Tanno-gelatin does 

 not appear to be a definite compound ; at least 

 it is difficult to obtain it as such : the preci- 

 pitate by infusion of galls consists, when care- 

 fully dried, of about 40 per cent, of tan and 60 

 of gelatin. When obtained by other astringents, 

 such as oak-bark, catechu, and kino, it differs 

 in the relative proportion of its components 

 and in its other characters, and often contains 

 extractive matter. According to Sir H. Davy,* 

 100 parts of calf-skin thoroughly tanned by 

 infusion of galls increase in weight 64 parts ; 

 by strong infusion of oak-bark 34, and by 

 weak 17; by concentrated infusion of willow- 

 bark 34, and by dilute 15; and by infusion 

 of catechu 19. 



Mr. Hatcltett's researches have shewn that 

 gelatin is also precipitated by the varieties 



* Philos. Trans. 



of artificial tan, and that the compound thrown 

 down resembles in its leading characters the 

 tanno-gelatin of natural tan. The ultimate 

 composition of gelatin (pure isinglass) has 

 been quantitatively determined by Gay Lussac 

 and Thenard, with the following results : 



Atoms. Equiv. 



Nitrogen .1 14 



Carbon . 7 42 



Hydrogen . 7 7 



Oxygen . 3 24 



Theocy. Experiment. 



16.09 16.998 



48.28 47.881 



8.04 7.914 



27.59 27.207 



87 100.00 100.000 



As the combining proportion of gelatin has 

 not been accurately ascertained, its equivalent 

 number, as above given, is open to doubt, 

 but it is probably correct, and the theoretical 

 and experimental results closely correspond. 



( W. T. Brandc.) 



GENERATION, ORGANS OF, (Com- 

 parative Anatomy). Few subjects connected 

 with physiology have been investigated more 

 assiduously than that of the generation of ani- 

 mals ; and in none, perhaps, has the poverty of 

 our knowledge of the operations of nature been 

 more conspicuously exemplified. In studying 

 many functions of the animal economy, the 

 laws of chemistry and mechanics have been suc- 

 cessfully appealed to by the philosopher, and 

 their application to the operations of the animal 

 frame satisfactorily substantiated ; but in at- 

 tempting to explain the wonderful process by 

 which organized bodies are perpetuated, all the 

 resources of modern science have been found 

 totally inadequate to the task, and we are still 

 left to record facts and observations concerning 

 the structure of the organs appropriated to the 

 propagation of animals, without being in any 

 degree able to connect them with the results so 

 continually offered to our contemplation. In 

 taking a general survey of the animal kingdom, 

 we are at once struck with the infinite variety 

 of forms which it presents, adapted to an end- 

 less diversity of circumstances, and might expect 

 to meet with a corresponding dissimilarity in the 

 organization of the generative apparatus pecu- 

 liar to each : no such dissimilarity, however, 

 exists in nature, the modes of reproduction 

 conform to a few grand types, and the increasing 

 complexity of parts, apparent as we ascend to 

 higher classes, which it will be our business to 

 trace in this article, will be seen to depend 

 rather upon modifications in the arrangement 

 of secondary structures than upon any deviation 

 from the fundamental organization of the more 

 immediate agents. 



Without entering upon any discussion con- 

 cerning the theories which have from time to 

 time been advocated relative to spontaneous 

 generation, we shall divide all animals as re- 

 lates to the generative function into three great 

 classes, grouping together such as are 



1st. Fissiparous,in which the propagation of 

 the species is effected by the spontaneous divi- 

 sion of one individual into two or more, pre- 

 cisely resembling the original being. 



