The simplest method of determining the density of a solution is to get the weight of a definite volume thereof. This is conveniently accomplished by the use of a pyknometer. A pyknometer is any vessel capable of holding a definite volume of a liquid in a form suited to weighing. It may be a simple flask with a narrow neck distinctly marked, or a flask with a ground perforated stopper, which, when inserted, secures always the same volume of liquid contents. A very common form of pyknometer is one in which the central stopper carries a thermometer and the constancy of volume is secured by a side tubulure of very small or even capillary dimensions, which is closed by a ground glass cap. The apparatus may not even be of flask form, but assume a quite different shape as in Sprengel’s tube. Pyknometers are often made to hold an even number of cubic centimeters, but the only advantage of this is in the ease of calculation which it secures. As a rule, it will be found necessary to calibrate even these, and then the apparent advantage will be easily lost. A flask which is graduated to hold fifty cubic centimeters, may, in a few years, change its volume at least slightly, due to molecular changes in the glass. Some of the different forms of pyknometers are shown in the accompanying figures. In use the pyknometer should be filled with pure water of the desired temperature and weighed. From the total weight the tare of the flask and stopper, weighed clean and dry, is to be deducted. The remainder is the weight of the volume of water of the temperature noted, which the pyknometer holds. The weight of the solution under examination is taken in the same way and at the same temperature, and thus a direct comparison between the two liquids is secured. _Example._—Let the weight of the pyknometer be 15.2985 grams. and its weight with pure water at 15°.5 be 26.9327 ” Then the weight of water is 11.6342 ” The weight filled with the sugar solution is 28.3263 ” Then the weight of the sugar solution is 13.0278 ” The specific gravity of the sugar solution is therefore, 13.0278 ÷ 11.6342 = 1.1198. For strictly accurate results the weight must be corrected for the volume of air displaced, or in other words, be reduced to weights in vacuo. This however is unnecessary for the ordinary operations of agricultural analysis. If the volume of the pyknometer be desired, it can be calculated from the weight of pure water which it holds, one cubic centimeter of pure water weighing one gram at 4°. The weights of one cubic centimeter of water at each degree of temperature from 1° to 40°, are given in the following table: TABLE SHOWING WEIGHTS OF ONE CUBIC CENTIMETER OF PURE WATER AT TEMPERATURES VARYING FROM 1° TO 40°. Weight, Weight, Temperature. Gram. Temperature. Gram.