What does latent heat mean? we hear the reader inquire. Let us try to explain it in simple language. Arago pronounced Black's experiment revealing it as one of the most remarkable in modern physics. Water passed as an element until Watt found it was a compound. Change its temperature and it exists in three different states, liquid, solid, and gaseous--water, ice and steam. Convert water into steam, and pass, say, two pounds of steam into ten pounds of water at freezing point and the steam would be wholly liquified, _i.e._, become water again, at 212 deg., but the whole ten pounds of freezing water would also be raised to 212 deg. in the process. That is to say two pounds of steam will convert ten pounds of freezing water into boiling water, so great is the latent heat set free in the passage of steam to lower temperatures at the moment when the contact of cold surfaces converts the vapor from the gaseous into the liquid state. This heat is so thoroughly merged in the compound that the most delicate thermometer cannot detect a variation. It is undiscoverable by our senses and yet it proves its existence beyond question by its work. Heat which is obtained by the combustion of coal or wood, lies also in water, to be drawn forth and utilised in steam. It is apparently a mere question of temperature. The heat lies latent and dead until we raise the temperature of the water to 212 deg., and it is turned to vapor. Then the powerful force is instantly imbued with life and we harness it for our purposes. The description of latent heat which gave the writer the clearest idea of it, and at the same time a much-needed reminder of the fact that Watt was the discoverer of the practically constant and unvarying amount of heat in steam, whatever the pressure, is the following by Mr. Lauder, a graduate of Glasgow University and pupil of Lord Kelvin, taken from "Watt's Discoveries of the Properties of Steam." It is well to distinguish between the two things, Discovery and Invention. The title of Watt the Inventor is world-wide, and is so just and striking that there is none to gainsay. But it is only to the few that dive deeper that Watt the Discoverer is known. When his mind became directed to the possibilities of the power of steam, he, following his natural bent, began to investigate its properties. The mere inventor would have been content with what was already known, and utilised such knowledge, as Newcomen had done in his engine. Watt might have invented the separate condenser and ranked as a great inventor, but the spirit of enquiry was in possession of him, and he had to find out all he could about the _nature_ of steam.