1890 - 91 .] Dr Haycraft on Specific Gravity of the Blood. 253 
My first attempt to estimate the specific gravity of blood was by 
quite another method. While watching a globule of blood slowly 
descend in a cylindrical vessel filled with oil, it occurred to me 
that by using drops of the same size, their specific gravity could 
easily be determined, for the higher their specific gravity the 
quicker would they fall. By the rate of fall, I found that the 
specific gravity could be determined with the greatest accuracy, but 
inasmuch as the viscosity of the oil, and therefore the rate of 
fall varies with the temperature, this must either remain constant 
or a correction made for it. As the viscosity varies considerably 
with even small changes of temperature I abandoned the method, 
as one incapable of clinical application, though it is, I find, highly to 
be recommended for laboratory purposes, where the temperature 
factor can be kept strictly under control. Having spent some time 
on this method I did not like to relinquish the subject, and the 
plan already described in this paper was worked out. It occurred 
to me that if I could obtain two fluids, both of which had a different 
surface tension from blood, one of which procured a higher, the 
other a lower specific gravity than blood itself, I could mix them 
until I hit off the exact specific gravity of any particular drop I 
might wish to examine. This is of course the principle of the 
method already detailed in this paper, but I was for some time 
unable to carry it out in practice, more especially as such fluids 
must be very mobile in order readily to mix with each other, and 
mobile fluids of high specific gravity are not very numerous. I 
first tried mixtures of chloroform and paraffin, one mixture having 
the sp. gr. 1*070, and the other sp. gr. 1*020. This plan did not 
succeed at all, for the mixture affects the density of the blood itself, 
a globule of blood, say of slightly lighter specific gravity than a 
given mixture of chloroform and paraffin, on its first immersion 
floating on the surface, but after a few seconds acquiring density 
and sinking in the fluid. I then tried mixtures of chloroform and 
toluol (chloroform, sp. gr. 1*498 ; toluol, sp. gr. 0*8706), and this 
time with success, the specific gravity of the blood remaining 
constant in the mixture. This method succeeds admirably, but 
the mixtures are apt to lose specific gravity on keeping, for the 
chloroform is very volatile, boiling at 61° C. I then sought for 
another mobile fluid having a higher specific gravity and a high 
