SULPHURIC ACID.

Table showing the percentage, by Weight, of Real Acid (H₂SO₄) in Aqueous Solutions of Sulphuric Acid of varying Specific Gravity. Temperature, 15° C.

1.838	100.0	1.568	66.0	1.247	33.0
1.840	99.0	1.557	65.0	1.239	32.0
1.841	98.0	1.545	64.0	1.231	31.0
1.841	97.0	1.534	63.0	1.223	30.0
1.840	96.0	1.523	62.0	1.215	29.0
1.838	95.0	1.512	61.0	1.206	28.0
1.836	94.0	1.501	60.0	1.198	27.0
1.834	93.0	1.490	59.0	1.190	26.0
1.831	92.0	1.480	58.0	1.182	25.0
1.827	91.0	1.469	57.0	1.174	24.0
1.822	90.0	1.458	56.0	1.167	23.0
1.816	89.0	1.448	55.0	1.159	22.0
1.809	88.0	1.438	54.0	1.151	21.0
1.802	87.0	1.428	53.0	1.144	20.0
1.794	86.0	1.418	52.0	1.136	19.0
1.786	85.0	1.408	51.0	1.129	18.0
1.777	84.0	1.398	50.0	1.121	17.0
1.767	83.0	1.388	49.0	1.113	16.0
1.756	82.0	1.379	48.0	1.106	15.0
1.745	81.0	1.370	47.0	1.098	14.0
1.734	80.0	1.361	46.0	1.091	13.0
1.722	79.0	1.351	45.0	1.083	12.0
1.710	78.0	1.342	44.0	1.075	11.0
1.698	77.0	1.333	43.0	1.068	10.0
1.686	76.0	1.324	42.0	1.061	9.0
1.675	75.0	1.315	41.0	1.053	8.0
1.663	74.0	1.306	40.0	1.046	7.0
1.651	73.0	1.297	39.0	1.039	6.0
1.639	72.0	1.289	38.0	1.032	5.0
1.627	71.0	1.281	37.0	1.025	4.0
1.615	70.0	1.272	36.0	1.019	3.0
1.604	69.0	1.264	35.0	1.013	2.0
1.592	68.0	1.256	34.0	1.006	1.0
1.580	67.0

APPENDIX B.

ESTIMATION OF SMALL QUANTITIES OF GOLD.[124]

In the case of small buttons of gold the weight can be determined more easily and accurately by measuring with the help of a microscope than by the actual use of a balance. Moreover, the method of measurement is applicable to the determination of quantities of gold too minute to affect even the most delicate balance.

For quantities of gold of from .5 to .005 milligram a microscope with 1/2 inch objective and B eyepiece is suitable. The measurements are made with the help of a scale engraved (or, better, photographed) on a circular piece of glass which rests on the diaphragm of the eyepiece. This scale and the object upon the stage can be easily brought into focus at the same time. The button of gold obtained by cupelling is loosened from the cupel by gently touching with the moistened point of a knife; it generally adheres to the knife, and is then transferred to a glass slide. The slide is placed on the stage of the microscope, illuminated from below; and the button is brought into focus, and so placed that it apparently coincides with the scale. The diameters in two or three directions (avoiding the flattened surface) are then read off: the different directions being got by rotating the eyepiece. The mean diameter is taken. The weight of the button is arrived at by comparing with the mean diameter of a standard prill of gold of known weight. The weights are in the proportion of the cubes of the diameters. For example, suppose a prill has been obtained which measures 12.5 divisions of the scale, and that a standard prill weighing 0.1 milligram measures 11.1 divisions. The weight will be calculated as follows:

11.1³ : 12.5³ :: 0.1 : x

	0.1×12.5×12.5×12.5
x =	——————————	= 0.143 milligram.
	11.1×11.1×11.1

The calculations are simplified by the use of a table of cubes. The standard prills used in the comparison should not differ much in size from the prills to be determined. They are prepared by alloying known weights of gold and lead, so as to get an alloy of known composition, say one per cent. gold. Portions of the alloy containing the weight of gold required (say 0.1 milligram) are then weighed off and cupelled on small smooth cupels, made with the finest bone-ash. Care must be taken to remove the cupels as soon as cupellation has finished. Several standard prills of the same size should be made at the same time, and their mean diameter calculated. The lead for making the gold-lead alloy is prepared from litharge purified by reducing from it about 10 per cent. of its lead by fusion with a suitable proportion of flour; the purified litharge is powdered, mixed with sufficient flour and reduced to metal.

In determining the gold contained in small buttons of silver-gold alloy obtained in assaying (and in which the silver is almost sure to be in excess of that required for parting), transfer the button from the cupel to a small clean porcelain crucible; pour on it a drop or two of nitric acid (diluted with half its bulk of water), and heat gently and cautiously until action has ceased. If the residual gold is broken up, move the crucible so as to bring the particles together, so that they may cohere. Wash three or four times with distilled water, about half filling the crucible each time and decanting off against the finger. Dry the crucible in a warm place; and when dry, but whilst still black, take the gold up on a small piece of pure lead. Half a grain of lead is sufficient, and it is best to hold it on the point of a blunt penknife, and press it on the gold in the crucible. The latter generally adheres. Transfer to a small smooth cupel and place in the muffle. When the cupellation has finished, the button of gold is measured as already described.