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CHAPTERS FOR STUDENTS. 
two globular flasks, fitted with india-rubber stoppers and a piece of glass tubing 
to connect them. The liquid may be introduced into a, and heat applied till 
steam issues from the extremity of the tube, when b may be attached and 
securely closed. So long as b is kept cool by a stream of water, a very little 
heat applied to a will suffice to keep up the ebullition. 
27. The temperature at which a liquid boils varies 
slightly, according to the nature of the vessel in which 
the operation is performed ; but the temperature of 
the vapour given off is uniform, provided the atmo¬ 
spheric pressure remains the same. Boiling-points 
are, therefore, taken by allowing the bulb of a ther¬ 
mometer to approach very near to the surface of the 
boiling liquid, but not to dip into it. All preceding 
remarks upon vapours hold good only so long as 
they are in contact with an excess of the liquid from which they have been 
formed. Dry vapours are influenced by heat, in the same manner as the nomi¬ 
nally permanent gases. 
28. When heat has been generated in a body, or has been received from some 
other body, that portion which goes to raise its temperature, or which consti¬ 
tutes what is sometimes called its sensible heat, is disposed of in one or more 
of several ways. 
Imagine a red-hot ball supported upon an iron tripod. Its temperature 
rapidly falls as heat passes out of it. The heat travels away by three different 
routes; the tripod “conducts” a considerable portion away to the table or 
floor ; placing the hand or a thermometer a few inches from the side of the ball, 
a certain portion will be found thrown off by “ radiation,” whilst a larger 
amount is carried upwards by “ convection ” as well as by radiation. 
29. Conduction of heat is effected most perfectly by solids, and of these by the 
metals. Liquids conduct very imperfectly, and gases so slightly as to be im¬ 
perceptible. 
Radiation of heat takes place through space, in straight lines, and in all direc¬ 
tions equally. 
Convection (from conveho ) means convejdng. The air in eontact with the 
hot surface receives heat,—its temperature is raised, 
it expands, rises, and when it comes in contact with 
the hand or thermometer above, gives up the heat it has 
received, and returns to its former state. Convection 
may easily be witnessed by applying heat to a test- 
tubeful of liquid, holding in suspension a precipitate. 
The currents established will be visible by the motion 
of the precipitate in the direction of the arrows. 
Radiant heat when it strikes upon a surface, whether 
solid, liquid, or gaseous, may suffer various modifica¬ 
tions. It may be reflected, that is castoff again, with¬ 
out sensible change or diminution. Or it may be 
absorbed; that is, it may warm the body and change 
its volume or physical state. Or it may be trans¬ 
mitted by or through the body without changing its 
temperature or condition. 
CHEMICAL NOTES TO THE PHARMACOPOEIA ( continued ). 
N.B. The formulae printed in the Pharmacopoeia in thick type will be those 
only which are referred to in the following pages. The formulae in thinner 
