The water of the bath is kept in brisk ebullition for thirty minutes, any evaporation from the flask being replaced by the addition of boiling distilled water. The stopper should be kept for a few minutes before use in hot distilled water and is then inserted, the flask removed, wiped dry, and, after it is nearly cooled to room temperature, placed in the balance and weighed when balance temperature is reached. A convenient size of holder will enable the analyst to use eight or ten flasks at once. The temperature at which water boils in each locality may also be determined; but unless at very high altitudes, or on days of unusual barometric disturbance the variations will not be great, and will not appreciably affect the results. =51. Alternate Method of Estimating the Weight of Water in Flasks.=—Formulas for calculating the volume _V_, in cubic centimeters, of a glass vessel from the weight _P_ of water at the temperature _t_ contained therein, and the volume _Vʹ_ at any other temperature _t’_ are given by Landolt and Börnstein.[28] They are as follows: _p_ _V_ = _P_ --- _d_ _p_ _Vʹ_ = _P_ ---- [1 + _γ_(_tʹ_- _t_)] _d_ _p_ = weight (in brass weights) of one cubic centimeter H₂O in vacuo. This is so nearly one gram that it will not affect the result in the fifth place of decimals and may therefore be disregarded. Hence the formula stands: 1 _Vʹ_ = _P_ ---- [1 + _γ_(_tʹ_-_t_)]; in which _d_ _d_ = density of water at temperature _t_. _γ_ = 0.000025, the cubical expansion coefficient of glass. From this volume the weight of the water may be readily obtained by referring to tables 13, 14 and 15_a_ in Landolt and Börnstein’s book. =52. Example Showing Determination of Specific Gravity of a Fat.=—The flask is emptied of its water, rinsed with alcohol and ether, and dried again for a few minutes at 100°. It is then filled with the dry, hot, fresh-filtered fat, which should be entirely free from air bubbles. The stoppered flask is then replaced in the water-bath, kept for thirty minutes at the temperature of boiling water, removed, and treated as above. The weight of fat having been determined, the specific gravity is obtained by dividing it by the weight of water previously found. _Example._ Grams. Weight of flask, dry 10.0197 Weight of flask, plus water 37.3412 Weight of water 27.3215 Weight of flask, plus fat 34.6111 Weight of fat 24.5914 Specific gravity = 24.5914 ÷ 27.3215 = 0.90008. The weight of the flask dry and empty and the weight of water at 99° to 100° contained therein may be used constantly if great care be taken in handling and cleaning the apparatus.