The thirst that not unfrequently accompanies exercise is due to the great escape of water from the skin which has been already alluded to. This liberation of moisture, being also accompanied, as already explained, by a large excretion of the chlorides and, perhaps, by other salts. Dr Parkes advises the use of an additional supply of chloride of sodium to the diet of those taking much exercise; he suggests that probably potassium chloride and phosphate might be added with advantage.

The evaporation from the skin has the effect of reducing the bodily temperature and rendering it equable. This temperature, however, falls very rapidly after exertion is over; and hence at this time it is always advisable to guard against the chance of a chill by covering the body over. Flannel forms the best protection. Keeping the skin clean by daily ablution greatly aids in the escape of fluid during exercise.

The large amount of carbonic anhydride given off by the lungs during bodily exercise explains the advantages of open air exercise, and why walking in the fresh air produces such excellent effects in some forms of dyspepsia. This increased exhalation of carbonic anhydride also points to the importance of thorough ventilation when indoor exercise is taken, particularly by large bodies of men or women, as in riding schools and on the treadmill. The mortality amongst miners, whose labour is performed in confined and ill-ventilated spaces is very great. According to Mr Simon, with the exception of those who work in the well-ventilated mines of Durham and Northumberland, the 300,000 miners in England break down prematurely from bronchitis and pneumonia, caused by the atmosphere in which they are compelled to work.

EXPAN′SION. All substances, solid, liquid, and gaseous, when chemical change does not take place, expand by heat, and contract by cold. In some of them this property occurs in a greater degree than in others, but is constant for the same substance under the same circumstances. The chemist avails himself of this property in the construction of his thermometer; the wheelwright, in fixing on the tire of his wheels; the engineer, in restoring to the perpendicular the leaning walls of buildings, &c.

This expansion by heat is of great importance in the manufactures, as allowance has to be made of it in every purpose where metals are employed.

The following is a list of the expansion of the chief metals, &c., when heated from 32° to 212° Fahr., or from 0° to 100° Cent.:—

Substance.Expansion.
In bulk.In length.
Glass1 in 3841 in 1150
Platinum1 in 3771 in 1311
Steel1 in 3091 in 926
Iron1 in 2821 in 846
Gold1 in 2271 in 682
Copper1 in 1941 in 582
Brass1 in 1791 in 536
Silver1 in 1751 in 524
Tin1 in 1721 in 516
Lead1 in 1171 in 351
Zinc1 in 1131 in 340

Of the liquids, they expand as follows, when

heated from 0° to 100° Cent., or from 32° to 212° Fahr.:—

Mercury1 in 55 in bulk.
Water1 in 21 in bulk.