of hydrogen bromide (HBr = 81) will diffuse under the same (iii) Calculate the percentage composition of theine. (iv) 10 c.c. of hydrogen nitrate A 1.5 are warmed with copper and water, what volume of nitric oxide at 80° C. and 1000 m.m. is given off? == 3Cu + 8HNO2 = 3Cu 2NO2+ 4H2O + 2NO. 10 × 1.5 × 2 × 30 22.32 8 x 63.3 2 X × (1+ a 80) × 760 1000' 11391.3 litres. 5.7045 (v) 0.5 gram of aniline, when burnt with cupric oxide, gave 36 c.c. of nitrogen measured over water at 15o C., under a pressure of 684 m.m. Find the percentage of nitrogen in the aniline. The tension of aqueous vapour at 15o C. is 12·7 m.m. (vi) 4376 grams of potassium chlorate are heated, what volume of oxygen measured over water at 25° C. and 684 m.m. is given off? The tension of aqueous vapour at 25° C. is 23-6 m.m. log 3.1766 1502 litres nearly, (vii) Determine the vapour-density of camphor from the following results due to Dumas. The volume of the globe was 295 c.c., and its increase in mass (W—w), when filled with camphor-vapour, was 0.708 grams. The temperature of the air was 13.5° C., and of the bath 239° C. The barometer stood at 742 m.m. 10 16 The formula of camphor is C,,H,,O, and hence its density is 76 by theory. (1 + ·00003 × 239) × ·0000896 × 742 295 × L. C. A. (1 + ·00003 × 13·5) × (1 × a239) × 760 6 If the volume of the globe at 13.5° C. be 1 it becomes at 2.1411' log of mass of an equal volume of by Hence camphor vapour is 5.32 times heavier than air. *THE REDUCTION OF EXPERIMENTAL (35) THE DIFFERENT KINDS OF ERRORS. An experiment is always liable to errors caused by mistakes, such as misreading the value of a weight in the pan of a balance, or the number marked on the graduation of a measure. Constant errors caused by imperfections in the instruments or in the observer also occur, such as a weight not really being of the value marked upon it, or an observer constantly adding an excess of a precipitant from inability to detect the first trace of a precipitate. Errors of this sort can only be lessened or removed by practice on the part of the observer, by care in testing the instruments used, and by varying the form of the experiments. But besides these every experiment is affected by socalled accidental errors, which arise from momentary changes in the instruments, or from the want of absolute accuracy in the eye or hand of the observer. Thus, in reading a standard barometer the same result is rarely obtained three times in succession. Accidental errors may however be to some extent corrected by repeating an experiment a sufficient number of times, for it is probable, that if a large number of observations of exactly the same kind be made, about half will be in excess and half in defect of the true value. Hence if the |