78 DR. YELLOLY'S REMARKS ON THE 



of a rat by a medical student some years since, and also in a calculus taken 

 from the bladder of a pig. I have since found, that concretions of a similar 

 description and form, are by no means uncommon in the former animal. 



Of the triple phosphate, or ammoniaco-magnesian phosphate. 



This substance is rarely found in its simple state, except as minute, trans- 

 parent crystals, deposited between other laminae. The nearest approach to it 

 seems to be in the irregular white, or yellowish, or brownish-white crystalliza- 

 tion, which is not unfrequently found on the surface of the mixed phosphates. 

 This crystallization, I have always found to contain a small quantity of lime ; 

 and it must therefore, I presume, be considered, according to the division 

 adopted by Dr. Wollaston, as belonging to the fusible calculi, or those con- 

 sisting of the mixed phosphates, but possessing, perhaps, the smallest quantity 

 of lime, which enters into the composition of this form of calculus. Dr. Prout 

 is inclined to view it as having some unknown, but regular proportion of the 

 two sets of ingredients. The varying proportions, of the two phosphates seem, 

 indeed, (as Dr. Marcet very judiciously observes,) to communicate every 

 degree of fusibility to the calculi which are composed of them. 

 •*^The usual mode of detecting minute quantities of the triple phosphate, is, I 

 believe, that recommended by Dr. Wollaston, of observing the formation of a 

 white line, by the deposition of the ciystals of that substance, on any part of the 

 glass vessel containing them, which has been rubbed by a glass tube, or other 

 pointed instrument. I have employed, however, what appears to me a still 

 more ready mode of ascertaining the formation of the triple phosphate, by 

 placing the fluid expected to contain it, in a watch-glass, in the field of a com- 

 pound microscope of moderate power. The triple crystals are thus capable of 

 being observed at the period of their earliest formation ; and their gradual in- 

 crease of size, and union with each other in various accidental ways, but mostly 

 in a stellated form, come within immediate view, and form an interesting sub- 

 ject of observation. — It is the more desirable to have a ready and uneqtiivocal 

 mode, of determining the existence of minute quantities of the ammoniaco- 

 magnesian phosphate, in the examination of animal bodies, as the production 

 of that substance affords a means, to which it is difficult to find a limit, of as- 

 certaining the existence of ammonia, magnesia, and phosphoric acid. 



I 



