The _correction for temperature_:--Multiply the volume by 273 and divide by the temperature (in degrees Centigrade) added to 273. For all three corrections the following rules hold good. _To reduce to 0° C. and 760 mm. dry._ Volume × 0.3592 × (Pressure-tension) Corrected volume = -------------------------------------- Temperature + 273 To find the volume, which a given volume under standard conditions would assume, if those conditions are altered. Volume × 2.784 × (Temperature + 273) Resulting volume = ------------------------------------ Pressure - tension As an example, we will suppose that it is desired to enclose in the apparatus referred to on p. 45, a volume of air, which, when dry (at 0° C. and 760 mm.), shall measure 100 c.c., whilst the actual temperature is 15° C., and the pressure 750 mm. The second formula is the one to be used, and we get 108.7 c.c. 100 c.c.×2.784×288 Required volume = ---------------------- 750-12.7 80179.2 = ------- 737.3 = 108.7 c.c. FOOTNOTES: [4] 100-35.6 = 64.4. CHAPTER V. WEIGHING AND MEASURING. ~Weighing.~--The system of weights and measures which we have adopted is the French or metric system; in this the gram (15.43 grains) is the unit of weight; the only other weight frequently referred to is the milligram, which is 0.001, or 1/1000 gram. The unit of volume is the cubic centimetre, which is approximately the volume of 1 gram of water, and which thus bears to the gram the same relation as grain-measures bear to grains. It is usual to write and even pronounce cubic centimetre shortly as c.c., and the only other denomination of volume we shall have occasion to use is the "litre," which measures 1000 c.c., and is roughly 1-3/4 pints. The weights used are kept in boxes in a definite order, so that the weights on the balance can be counted as well by noting those which are absent from the box as by counting those present on the scale-pan. The weights run 50, 20, 10, 10, 5, 2, 1, 1 and 1 grams, and are formed of brass. The fractions of the gram are generally made of platinum or of aluminium, and are arranged in the following order:--0.5, 0.2, 0.1, 0.1, and 0.05, 0.02, 0.01, 0.01. These may be marked in this way, or they may be marked 500, 200, 100, 100, 50, 20, 10, 10; the 500 meaning 500 milligrams. Some makers send out weights in the series 50, 20, 20, 10, &c.