contained in a beaker, and slowly heat the vessel till the thin opaque cylinder of solid fat melts and becomes transparent: a delicate thermometer placed in the water indicates the point of change to the fifth of a degree. Remove the source of heat, and note the congealing-point of the substance; it will be identical with or close to the melting-point.

The following melting-points of officinal substances are given in the British Pharmacopoeia:

The order of fusibility of a few of the metals is as follows:

50

122

43.3

110

(about)

38

100

38

100

(not under)

65.5

150

60

140

SPECIFIC GRAVITY.

All bodies attract each other, the amount of attraction being in direct proportion to the quantity of matter of which they consist, and in inverse proportion to the square of their distances. This is gravitation. Weight is the effect of gravity, being the excess of the attraction which the earth has for a body over that which a body has for the earth. Specific weight is the definite or precise weight of a body in relation to its bulk; it is more usually termed specific gravity *. The comparative heaviness of solids and liquids is conventionally expressed in relation to water: they are considered as being so much lighter or heavier than water. Thus, water being regarded as unity-1, the relative weight, or specific gravity, of ether is represented by the figures 720 (it is nearly three-fourths, 750 the weight of water), oil of vitriol by 1.843 (it is nearly twice, 2.000 as heavy as water). The standard of comparison for gases is usually air, but sometimes hydrogen.

SPECIFIC GRAVITY OF LIQUIDS.

Procure any small bottle holding from 100 to 1000 grains, and having a narrow neck; counterpoise it in a delicate balance; fill it to about halfway up the neck with pure distilled water having a temperature of 60° F.; ascertain the weight of the water, and add or subtract a drop or two, so that the weight shall be a round number of grains; mark the neck by a diamond or file-point at the part cut by the lower edge of the curved surface of the water. Consecutively fill up the bottle to the neck-mark with several other liquids, cooled or warmed to 60° F., first rinsing out the bottle once or twice with a small quantity of each liquid, and note the weights; the respective figures will represent the relative weights of equal bulks of the liquids. If the capacity of the

* Density is sometimes improperly regarded as synonymous with specific gravity. It is true that the density of a body is in exact proportion to its specific gravity; but the former is more correctly the comparative bulk of equal weights, while specific gravity is the comparative weight of equal bulks.

bottle is 10, 100, or 1000 grains, the resulting weights will, without calculation, show the specific gravities of the liquids; if any other number, a rule-of-three sum must be worked out to ascertain the weight of the liquids as compared with 1 (or 1·000) of water. Bottles conveniently adjusted to contain 250, 500, or 1000 grains, or 100 or 500 grammes of water, when filled to the top of their perforated stopper, and other forms of the instrument, are sold by all chemical-apparatus makers.

The Balance. The balance used in the foregoing and other quantitative operations must be accurate and sensitive. The points of suspension of the beam and pans should be polished steel knife-edges, working on agate planes. It should turn easily and quickly, without too much oscillation, to or of a grain,

or of a milligramme, when 1000 grains, or 50 or 60 grammes, are placed in each scale. The beam should be light but strong, capable of supporting a load of 1500 grains or 100 grammes; its oscillations are observed by help of a long index attached to its centre, and continued downwards for some distance in front of the supporting pillar of the balance. The instrument should be provided with screws for purposes of adjustment, a mechanical contrivance for supporting the beam above its bearings when not in use or during the removal or addition of weights, spirit levels to enable the operator to give it a horizontal position, and be enclosed in a glass case to protect it from dust. It should be placed in a room the atmosphere of which is not liable to be contaminated by acid fumes, in a situation free from vibration, and a vessel containing lumps of quicklime should be placed in the case to keep the enclosed air dry and prevent the formation of rust on the steel knife-edges or other parts. During weighing the doors of the balance should be shut, in order that currents of air may not unequally influence the pans.

The Weights.-These should be preserved in a box having a separate compartment for each. They must not be lifted directly with the fingers, but by a small pair of forceps. If grain-weights, they should range from 1000 gr. togr., a weight being fashioned of gold wire to act as a "rider 29 on the divided beam, and thus indicate by its position 100ths and 1000ths of a grain. From to 10 grs. the weights may be of platinum; thence upwards to 1000 grs., of brass. The relation of the weights to

each other should be decimal. Metric decimal weights may range from 100 grammes to 1 gramme, of brass, and thence downwards to 1 centigramme, of platinum; a gold centigramme rider being employed to indicate milligrammes and tenths of a milligramme. The metric system of weights is greatly to be preferred to the British, the relation of the metric weights of all denominations to measures of length,. capacity, and surface being so simple as to be within the comprehension of the merest child; while under the British plan, the weights have no such relation, either with each other or with the various measures. Moreover the metric system is in perfect harmony with the universal method of counting; it is a decimal system.

It is perhaps impossible to realize, much more express, the advantages we enjoy from the fact that in every country of the world the system of numeration is identical. That system is the decimal. Whatever language a man speaks, his method of numbering is decimal; his talk concerning number is decimal; his written or printed signs signifying number are decimal. With the figures 1, 2, 3, 4, 5, 6, 7, 8, 9, 0 he represents all possible variation in number, the position of a figure in reference to its companions alone determining its value, a figure on the left hand of any other figure in an allocation of numeral symbols (for example, 1867) having ten times the value of that figure, while the figure on the right hand of any other has a tenth of the value of that other. When the youngest apprentice is asked how many units there are in 1867, he smiles at the simplicity of the question, and says 1867. How many tens? 186, and 7 over. How many hundreds? 18, and 67 over. How many thousands? 1, and 867 over. But if he is asked how many scruples there are in 1867 grains, how many drachms, how many ounces--he brings out his slate and pencil. And so with the pints or gallons in 1867 fluid ounces, or the feet and yards in 1867 inches, or the pence, shillings, and pounds in 1867 farthings; to say nothing of cross questions, such as the value of 1867 articles at 9s. 6d. per dozen, or of the perplexity caused by the varying values of several individual weights or of measures of length, capacity, and surface in different parts of the country. What is desired is, that there should be an equally simple decimal relation among weights and measures and coins as already universally exists among numbers. This condition of things having already been introduced into other countries, there is no good reason why it should not be accomplished in this.

WEIGHTS AND MEASURES OF THE METRICAL

SYSTEM.

(From the British Pharmacopoeia of 1867.)

WEIGHTS.

the thousandth part of one grm. or 0.001 grm.

1 Centigramme the hundredth

1 Milligramme

1 Decigramme

the tenth

weight of a cubic centimetre of
water at 40° C.

1 Decagramme = ten grammes

1 Hectogramme=one hundred grammes 1 Kilogramme =one thousand grammes

MEASURES OF CAPACITY.

1 Millilitre = 1 cub. centim. or the mea. of 1 gram. of water.

1 Millimetre

MEASURES OF LENGTH.

the thousandth part of one metre or 0·001 metre.

the ten-millionth part of a quarter of the meridian of the earth.