Heat by Conduction in Bone, Brain-tissue, and Skin. 179 



The first important points of the methods, are, therefore, the deter- 

 mination and regulation of the differences between the temperatures 

 of the air and the water. These differences were determined by ther- 

 mometers and thermo-piles (the latter being included in a circuit distinct 

 from that of the pile used in testing for conductivity, and having their 

 own galvanometer) placed in and near the water, the thermo-piles 

 giving differences of o- 02 C. In practice it was found that, with care 

 and patience, a difference of about 0°*125 0. between the air and water 

 could be pretty steadily maintained long enough for the purposes of 

 the experiments. It is, however, as a rule, better to reverse the 

 ordinary order of things, and to take heat from the hone instead of 

 furnishing heat to it, that is to say, it is better to have the temperature 

 of the water lower by the desired amount than that of the air, than to 

 have it higher, for the temperature of the water is more easily main- 

 tained at a point differing slightly from the temperature of the air 

 when the former is the lower of the two. If the temperature of the 

 room in which the experiment is made be carefully watched, we may 

 be certain that the temperature of the water will not exceed that of 

 the air; the principal difficulty will be to keep the temperature of the 

 water from falling too much below that of the air, and this end is best 

 attained by withdrawing by suction, through a long tube held in the 

 mouth, a small quantity of the liquid, and then returning it after a 

 longer or shorter stay in the mouth.* A little practice will enable one 

 to graduate, in this simple manner, with great nicety, the tempera- 

 ture of a small mass of water. The amount of water usually employed 

 was about one quart contained in an earthen vessel, exposing no 

 more surface of water to the air than was necessary for the intro- 

 duction of the different appliances used in the experiments. 



We have next to attend to the manner of applying the thermo- 

 pile to the surface of the substance examined, and the precautions 

 necessary in so doing ; and here the two methods diverge, the one 

 being applicable to the case of bone, and the other to that of brain and 

 skin. We will consider each method in turn, taking first that which 

 concerns bone. 



Bone. 



To begin with, the closest possible contact between the face of the 

 pile and the bone must be aimed at. To this end, the surface of the 

 bone is filed smooth, and the face of the pile having been accurately 

 fitted to it, the two are closely and permanently attached to each other 

 by means of a thin layer of shellac varnish applied to the face of the 

 pile and to the surrounding ebonite casing. Firm and steady pressure 

 must be maintained until the shellac is quite dry, as the interposition 



* Care must be taken not to alter sensibly the level of the water by withdrawing 

 too large an amount, for reasons to be seen further on. 



N 2 



