CHAP. IV.] THE BLOOD. METHODS OF RESEARCH. 217 



finger were counted. By assuming that in the interval which had 

 elapsed between the venesection and the second enumeration the 

 volume of blood had become exactly the same as it had been at the 

 time of the first enumeration, and further that no formation of new 

 corpuscles had taken place in the same period, Malassez obtained data 

 for calculating the total mass of the blood. 



It is obvious however that these assumptions are altogether 

 unwarrantable, and if in one case they led to a result not far removed 

 from the truth, such was a mere result of chance. 



Medico-Legal Detection of Blood-Stains. 



Detection It not unfrequently happens that the medical 



of blood cells jurist is asked to decide whether a certain stain upon 



clothes, weapons, floors, &c. is a stain of blood. When 



recent, the identification of a blood- stain presents no 



difficulties. By moistening it with diluted glycerine of specific 



gravity 1025 and, after some time, expressing the liquid, we may 



obtain microscopic evidence of the presence of blood corpuscles ; 



when such is the case the observer may be able to state positively 



that the blood was or was not the blood of a mammal, but cannot 



venture upon any more definite expression of opinion. 



Chemical Whether successful or not in the detection of blood 



reaction of corpuscles, it is always desirable to obtain the chemical 

 chief blood proofs of the presence of blood ; and with proper treat- 

 constituents ment this is possible even with blood- stains of consider- 

 in stain. %k>\Q antiquity and of small size. 



We shall suppose that the observer is examining a cloth stained 

 with blood; having selected the particular stain which he wishes 

 to examine he may, with pencil, draw a circle around it and mark 

 the circle with a letter or number, for purposes of identification and 

 description. He then will proceed to cut out the stain and to 

 pass a thread through it; the blood-stained piece of cloth is then 

 suspended in a very small test-tube containing a few drops of 

 distilled water ; the size of the tube must depend upon the estimate 

 which the experimenter forms of the amount of blood in the stain. 

 The piece of stained cloth is left to soak for one or two hours, at the 

 end of which time the water will usually have acquired a more or less 

 distinctly red colouration. By the aid of the thread which had been 

 attached to it the little piece of cloth is now withdrawn from the water 

 and pressed with a small glass rod against the upper part of the test- 

 tube so as to squeeze out the liquid which it had imbibed. A small 

 quantity of the liquid may be examined in a small cell with the aid 

 of the microspectroscope ; but only when the examination is carried 

 on by a person who has by considerable practice familiarized himself 

 with the use of the instrument and with the various absorption 

 spectra of colouring matters. 



