THE BAKERS GUIDE

and corrosive properties. It is distinguished by the terms double and single, the latter being only half the strength of the former. The more concentrated acid, which is much stronger even than the double aqua fortis, is termed spirit of nitre.

Aqueous (aqua, water). A term in general use for designa- ting definite combinations with water. The term hydrate has long been employed for the same purpose. A prefix is used when there is more than one atom, as in £/#-aqueous, ter- hydrate.

Assaying. The chemical operation of ascertaining the quantity of any metal in an ore or mixture. It differs from analysis only in degree ; and is performed in the dry way, as by heat ; in the moist way, as by acids, and other re-agents ; or by both methods.

Assimilation. The conversion of the food into nutriment

Astringents. To bind and contract.

Atmosphere (vapour, a sphere). That volume of air which surrounds the earth.

1. Atmospheric Pressure is indicated by the length of a column of mercury. A mercurial column, 30 inches in length, presses on a given surface with the same force as the atmosphere in its ordinary state ; and hence the force of a 60-inch column is equal to the pressure of two atmospheres ; that of 1 5 inches to half an atmosphere ; that of one inch to i-3oth of the atmos- pheric pressure.

2. Atmospheres— two, three, &c. Multiplied pressures of air, arising from condensation, the ordinary pressure being about fifteen (147 lbs.) pounds on the square inch.

Atom (priv., to cut). An ultimate particle of matter, incapa- ble of further division.

Atomic Theory. A theory introduced for explaining the laws of definite proportions in chemical combinations. It is founded on the supposition that matter consists of ultimate indivisible particles, called atoms ; that these are of the same size and shape in the same body, but differ in weight in different bodies; and that bodies combine in definite proportions, with reference to those weights, which are hence called atomic weights. The main features of this theory are briefly : —

1. In bodies capable of assuming tbe gaseous form, the weight of the atom is obtained from the volume ; thus, water being composed of one volume of oxygen, united with two volumes (or one atom) of hydrogen, the relative weights will be, oxygen 8, hydrogen 1, and water 9.

THE BAKERS GUIDE,

2. In bodies which do not assume the gaseous form in their simple state, the weight of the atom is deduced from that of the compound; the weight of carbon, for instance, is obtained from that of carbonic acid gas, one volume of which weighs 22 times as much as our standard of unity; of these 22 parts, 16 are oxygen, leaving 6 to represent the primary molecule of carbon.

3. In the case of bodies which are incapable of assuming a gaseous form, either alone or in combination, the weight must be oDtained by analysis ; thus, marble, or the carbonate of lime, is found to be composed of 22 parts of carbonic acid, and 28 of lime : 28 therefore represents the atomic weight of lime.

4. The atomic weights are generally supposed to be related to one another by multiple ; hence, this is often called the law of multiples, or of combinations in multiple proportion. This will be easily seen by referring to the component parts of the following substances :

Nitrogen. Oxygen,

Nitrous oxide 14 8

Nitric oxide ... ... ... 14 16

Hyponitrous acid ... ... 14 24

Nitrous acid 14 32

Nitric acid 14 4°

See Books on Chemistry. Attemperator. A vessel for measuring the quantity and heat of water ; a tank fitted with hot and cold water, a thermom- eter showing outside the temperature of the water inside. There is also an indicator marked off in quarts and gallens, showing the quantity run in or out. A very useful tank to the baker, enabling him to mix water of different temperatures together, so that one part qualifies the other by combination, until he has the quantity and temperature he requires.

Attenuation (attenuo, to make thin). The lessening of weight, or of consistency ; emaciation. The term is applied to the process by which a fluid becomes of less specific gravity, as when it undergoes fermentation, and parts with carbonic acid.

Capillary Attraction. The power by which a liquid rises in a fine tube higher than the surface of the liquid which surrounds it.

Electrical Attraction. The property displayed by certain sub- stances of attracting certain others, on being rubbed.

Magnetic Attraction. The tendency of certain bodies, chiefly iron, towards the north pole of the earth and each other.

Attraction of CoJiesion. The tendency of the molecules of a body to cohere^ to form masses.

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