CHAPTERS FOR STUDENTS. 
651 
more than those portions of each subject which every pharmaceutical student 
ought to be acquainted with. They are compiled in the hope that they may 
be found useful to those to whom they are now offered. 
HEAT.* 
1. Heat is a term applied indiscriminately to a certain cause as well as to cer¬ 
tain effects. We say that we feel heat when we experience certain sensations ; 
but we also give the same name to the something which causes the sensations. 
In natural philosophy it is best to limit the use of the term to this hidden 
cause ; heat, then, is that which tends to produce change of temperature, and 
consequently, excess or deficiency of it to make us feel hot or cold. 
2. Heat is present wherever matter exists. Hot and cold are merely relative 
terms; the temperatures with which w r e are commonly concerned being in¬ 
cluded within a certain narrow compass, which forms only part of a long scale, 
to which there are practically no limits either way. 
3. The sensations of heat and cold depend, in some measure, upon the pre¬ 
vious condition of the part of the body subjected to the test. If the right hand 
grasp a piece of ice for a few minutes, and it be then plunged into water at the 
temperature of the room, the water will seem to be warm. If meanwhile the 
left hand has been plunged into warm water, the water just tried with the right 
hand and found warm, will to the left seem to be cold. 
4. It is necessary to distinguish between quantity of heat and change of tem¬ 
perature. Increase of heat is the cause of rise of temperature. Removal of heat 
is the cause of fall of temperature. 
5. Increase or diminution of heat gives rise either to change of physical state 
( i. e. solid to liquid or gaseous, or vice versa) or to change of temperature. The 
former will be referred to hereafter. The most striking acconqoaniment of the 
latter, that is, of change of temperature, is the expansion or contraction of 
volume which results. 
Examples such as the following, illustrate this statement:— 
A bar of iron may be selected of such a thickness that, when cold, it will just 
pass through a ring ; made very hot, it will no longer do so. 
In very hot weather clocks with ordinary pendula lose ; in cold weather they 
gain time. The reason is, that the pendulum is longer when warm, and 
shorter when cold. If made longer, its vibrations are slower ; if shorter, its 
vibrations are quicker. 
If cold water be poured into a flask to the brim, some of it will run over when 
the flask is placed upon a hot plate. Upon cooling again, there will be no 
longer sufficient water to fill the flask. 
An india-rubber ball, partly filled with air, will swell up and become com¬ 
pletely distended when placed near a fire. 
6 . The difference of bulk, caused by an equal change of temperature, i 3 
greater in liquid than in solid bodies, and greatest of all iu aeriform bodies 
(gases and vapours). A simple experiment will show this a3 well as, in a rough 
way, the amount of expansion or contraction. A needle with the point broken 
off. is introduced into a piece of tubing of small bore (e.g. coarse thermometer 
tubing), to which it fits, and after it a small quantity of mercury, in such a 
way that they enclose between them a column of air: let the three be of the 
same length. Close the tube at the end of the needle. 
Suppose them to have been adjusted at the temperature of freezing, and that 
* To be followed by Light, Electricity, Magnetism. 
2 U 2 
