A combination of the two gases, chlorine and oxygen, becomes solid at -75°, and the protoxide of nitrogen at -150°. Cyanogen, a compound of carbon and nitrogen--the base of prussic acid--is solidified at 30° below the zero of our thermometric scale. The well-known pungent compound, ammonia, so exceedingly volatile at common temperatures, is converted into a crystalline, translucent, white substance at the temperature of -103°. The difficulties which necessarily attend the exposure of a body to extreme cold and great pressure at the same time, appear to be the only obstacle to the condensation of oxygen, hydrogen, and nitrogen gases. A sufficient amount of condensation was, however, effected by Dr. Faraday, to lead him to the conclusion, arrived at also by other evidences, that hydrogen, the lightest of the ponderable bodies, partakes of the nature of a metal.[82] During the solidification of water by freezing, some remarkable facts may be noticed. Water, in cooling, gradually condenses in volume until it arrives at 40° Fahr., which appears to be the point of greatest density. From this temperature to that of 32°, the point at which it begins to solidify, its volume remains unchanged,[82] as crystallisation (freezing) begins, the bulk increases, the mass becomes specifically lighter, and it swims on the surface of the fluid. From 40° to 32° the particles of water must be taking up that new position which is essential to the formation of the solid--ice; and while this is taking place, every substance held in solution by the water is rejected. If we mix with water the deepest colouring matter--the strongest acid or the most acrid poison--they are each and all rejected during the process of freezing, and if the water has been kept in a state of agitation during the process--so that the liberated particles may not be mechanically entangled--the ice will be transparent, colourless, tasteless, and inert--the substances rejected being gathered together in the centre of the frozen mass in a state of intense concentration. In like manner, even the atmospheric air, which is always held in solution, is rejected, and hence the reason why all the ice which forms upon still ponds is full of air-bubbles, while the ice which is produced in agitated water is perfectly free from them. This in itself is a remarkable condition, the entire bearing of which is not clearly understood; but a still more singular fact has been discovered in intimate connection with the rejection of all matter from a freezing solution. Water, which in this way is freed entirely of air, will not boil at 212° F., the ordinary boiling point of water.