1896.] MICROSCOPICAL JOURNAL. 169 



Hammersclilag aud depends upon the well-known pliysi- 

 cal principle that a hody which will float inditl'erently in 

 a liquid is of the same specific gravity as tliat licpiid. 

 For tliis pur[)ose, two liquids arc taken, one of a higher 

 and the other of a lower specific gravity than that of the 

 blood, with neither of which it will mix. The necessary 

 apparatus consists of a hydrometer, hydrometer jar, 

 chloroform and benzole. 



In using this metliod, the finger is pricked and the 

 blood thus obtained is introduced into a mixture of chloro- 

 form and benzole in the hydrometer jar. The drop of 

 blood, since it will not mix with either chloroform or 

 benzole, retains its spherical form. If the drop sinks the 

 mixture is too light and must be made heavier by adding 

 chloroform. If it rises the mixture is too heavy and 

 must be made lighter by adding benzole. By carefully 

 adding one or the other a point is reached where the 

 drop of blood will neither rise nor sink, but will float in- 

 differently in the mixture. At this point the specific 

 gravity of the blood is the same as that of the mixture. 

 By means of the hydrometer we can obtain the specific 

 gravity of the mixture and thus at the same time that of 

 the blood. 



It is desirable to use a medium-sized drop of blood 

 and it is better not to divide this into several. Care 

 must be taken, however, to mix the liquids thoroughly by 

 stirring with the glass rod. In order to facilitate mixing, 

 it is well, when the liquid is too heavy, to add an excess 

 of benzole and bring it back to the desired point by add- 

 ing chloroform. The latter- being heavier, sinks and thus 

 mixes more readily with the mixture. 



We have found it convenient to obtain the blood from 

 the palmar surface of the middle finger of the left hand, 

 and have used, for this pupose, an ordinary sharp-pointed 

 steel pen with one nib broken off. A new pen may be 

 used for every test and should be sterilized by heat. The 



