"Dissolve 17 grams of mercuric chloride in about 300 c.c. of distilled water; dissolve 35 grams of potassium iodide in 100 c.c. of water; add the former solution to the latter, with constant stirring, until a slight permanent red precipitate is produced. Next dissolve 120 grams of potassium hydrate in about 200 c.c. of water; allow the solution to cool; add it to the above solution, and make up with water to one litre; then add mercuric chloride solution until a permanent precipitate again forms; allow to stand till settled, and decant off the clear solution for use; keep it in glass-stoppered blue bottles, and set away in a dark place to keep it from decomposing."

This reagent can readily be obtained from any chemist after the above prescription, and all that is required is to add a few drops to a test-glass full of the suspected water or brine, which will assume a yellow colour if ammonia is present at all, or a full brown if it is present in any quantity.

The curve of vapour tensions of anhydrous ammonia, showing graphically the pressure of the gas in pounds per square inch for various temperatures, is given in fig. 20.1 Its latent heat of vaporization for various temperatures is shown in the table at the end of the book, and it will be noticed that at atmospheric pressure its temperature is -30° F., and its latent heat of vaporization-that is to say, the amount of heat that it will have to absorb before it will turn from a liquid to a gas-is 573.69. this pressure one pound of the gas has a volume of 18.693, and the liquid a volume of 02374; while the weight of the vapour is 0535, and of the liquid 42.123. The

1 In the vertical column of pounds per square inch the bottom figure should be 10.

properties of the gas for other pressures and temperatures can readily be found from the table.

A glance at the curve will show how important it is from an economical standpoint to keep the pressure of the gas in the condenser as low as possible, and as this pressure is determined by the temperature of the condenser, it is essential for economical working to have ample cooling water at a low temperature. It will also be noticed that as the pressure of the gas falls the volume increases. At a pressure of 30 lbs. on the square inch the temperature is zero F. and the volume 9.028; at a pressure of 14:13 lbs. on the square inch the temperature is -30° F. and the volume 18.693, or more than double. This means that, owing to the increased volume of the gas, the compressor has, at the lower temperature, to take twice the number of strokes per minute, or, if the speed is kept constant, to be twice the size, in order to pass the same quantity of gas and do the same amount of work as it would were the higher temperature in use. For this reason, and from this point of view, it is preferable, where possible, to expand the ammonia gas directly into pipes placed in the cold rooms, or liquid to be cooled, instead of effecting a transmission of heat by first cooling and then circulating brine; there are, however, cases where this cannot be done, but of these more hereafter.

Ammonia Accidents-First Aid.-Ammonia is incapable of being respired, and will produce suffocation if inhaled in any quantity.

Several fatal accidents have at times been caused by the bursting of portions of an ammonia plant, and the consequent escape of large volumes of the gas into the rooms where workmen are employed. In emergencies of this sort the first action is naturally to extract any person who may be

overcome from the room with the least delay possible. If a sponge, towel, or lump of waste is saturated with water and held over the mouth and nose, a room charged with ammonia gas can be entered and the person overcome rescued. The first thing to do is, obviously, to send for the nearest doctor, but life may often be saved and pain lessened by at once administering a glass of beer or a dose of common vinegar or very weak acetic acid. Either of these will act as an antidote to the ammonia.

CARBONIC ANHYDRIDE.

This gas, commonly known by its chemical symbol of CO2, is probably the most familiar of all the gases. It is responsible for the sparkle of champagne, beer, and aerated waters, and exists in nearly all water. In its natural state it is a colourless, odourless gas, and, at a temperature of 32° F. and a pressure of 36 atmospheres, it can be converted into a colourless, odourless liquid.

When carbon is burned in oxygen, CO, is formed, but if the supply of oxygen is insufficient, carbonic oxide (CO), an extremely poisonous gas, is also produced. The latter will burn with a blue flame, and by its combustion produce CO2, but pure CO2 will neither originate nor support combustion. CO2 does not possess the affinity for water that is common to ammonia, but it will dissolve in about its own volume, forming carbonic acid (H,CO). The solution has a sharp acid taste, and will turn blue litmus paper red. The volume of CO, that water is capable of absorbing decreases as the temperature rises, as pointed out on page 167 (watersoftening). All CO2 can be expelled if the water is brought to boiling-point.

The Properties of CO,.-As a refrigerant CO, is very