The operation of liquid chlorine machines is exceedingly simple. After the cylinders have been connected, the cylinder valves are opened and the joints tested for leakage by holding a swab of absorbent cotton saturated with strong ammonia under them; a leakage is indicated by the appearance of white fumes of ammonium chloride. The control valve is then slightly opened and the auxiliary cylinder valves partially opened; whilst the pressure in the apparatus is slowly increasing the remainder of the joints are tested and if found to be tight, the cylinder valves are fully opened and the control valve opened to the desired amount. In the solution feed types the water required as solvent is turned on before the control valve is opened. Once the apparatus is working, no further attention is required, except for the regulation of the dosage in the manual control types, until the cylinders are replaced. When the stock of gas in the cylinders is almost depleted the pressure falls but it is always preferable to determine the stock by standing the cylinders on a platform scale and weighing at regular intervals. This also provides a check on the apparatus and can be utilised to check the operators. The accumulation of substances that impede the flow of gas is usually slow and is indicated by a gradual increase in the back pressure. The orifice is calibrated at 25 pounds back pressure and any deviation from this figure will show a discrepancy between the actual weight of chlorine evaporated and the amount calculated from the scale reading. Liquid chlorine is usually sent out by the manufacturers in steel cylinders which contain about 1.1 cubic feet of liquid or approximately 100 pounds (1 cu. ft. = 89.75 pounds).[A] [A] An effort is now being made to standardise cylinders of 150 lbs. capacity. For small installations only one cylinder is necessary but it is always preferable to connect more than one. When the flow of gas is rapid the temperature of the liquid chlorine falls and reduces the pressure. The effect of the fall in temperature, due to the latent heat of evaporation, can be partially overcome by using a larger number of cylinders; in addition a source of external heat should be provided that will maintain the temperature of the cylinders at a minimum of 80° F. This is a "sine qua non" for successful operation. The effect of the temperature upon the pressure in the cylinders is shown in Diagram VII. [Illustration: DIAGRAM VII CHLORINE GAS PRESSURES AT VARIOUS TEMPERATURES] In practice it is found impossible to utilise all the gas contained in the containers; when the cylinders are almost empty the pressure necessary for the operation of the regulating device cannot be obtained and full cylinders must be attached. When sufficient heat is provided the weight of chlorine in the cylinder can be reduced to 1 - 1-1/2 pounds before the tank pressure becomes too low.