MICROSCOPICAL TECHNIQUE, ETC. 1 65 
solvent. If oil has been used the paraffin is changed 
after some hours for fresh; after treatment with the 
volatile substances we strongly recommend the use of 
an air-pump, or a water-filter pump. When the tissue 
is thoroughly warmed in the paraffin, the vessel con¬ 
taining it in the paraffin is placed under the receiver of 
an air-pump, and the air exhausted ; bubbles will at 
once rise, and the exhaustion is to be kept up till no 
more bubbles rise; the tissue is then put into fresh 
paraffin. The same course is followed with the water- 
pump ; the tissue is in a tube of glass, and this tube 
is attached to the pump, and so left till no bubbles 
appear. Of course, the paraffin must be kept melted 
by the application of heat; but for a full account of 
these matters the reader is referred to the Journal of 
Pathology and Bacteriology , Vol. I., No. i, May, 1892. 
[Pentland, Edinburgh.] This system of exhaustion 
greatly accelerates the process of infiltration, but its 
chief advantage lies in the facts that it makes the pene¬ 
tration of the paraffin more certain and more complete, 
and that one can tell when the process is complete by 
the cessation of the bubbling. 
With regard to the foregoing, it may be noted that 
to ensure the dehydrating alcohols penetrating the 
tissues, it is well either to suspend the tissues in muslin 
bags in the alcohols, or to place at the bottom of the 
tubes small plugs of lightly-packed cotton-wool. If 
any failure occurs in the paraffin process it will almost 
certainly be due to imperfect dehydration. Some 
tissues, as lung, float on the alcohol and paraffin 
