It radiates from _all bodies_ in straight lines, and in all _directions_. It strikes most powerfully in _direct lines_. Its rays may be collected into a _focus_, just as the rays of the sun. It may be _reflected_ from a polished surface. It is more easily _conducted_ by some substances than by others. [Verse: "For my days are consumed like smoke, and my bones are burned as an hearth."--PSALM CII.] 87. _What is animal heat?_ Animal heat is derived from the slow combustion of _carbon_ in the blood of animals with the _oxygen_ of the air which the animals breathe. 88. _What is latent heat?_ Latent heat (or more properly _latent caloric_) is that which exists, in some degree, in all _bodies_, though it may be imperceptible to the _senses_. 89. _Is there latent caloric in ice, snow, water, marble, &c?_ Yes; there is some amount of _caloric_ in all substances. A blacksmith may hammer a small piece of iron until it becomes _red hot_. With this he may light a match, and _kindle the fire of his forge_. The iron has become more dense by the hammering, and it cannot again be heated to the same degree by similar means, until it has been exposed _in fire_, to _a red heat_. Is it not possible that, by hammering, the particles of iron have been driven closer together, and _the latent heat_ driven out? No further hammering will force the atoms nearer, and therefore no further heat can be developed. But when the iron has _again absorbed caloric_, by being plunged in a fire, it is again charged with latent heat. Indians produce _sparks_ by rubbing together _two pieces of wood_. Two pieces of ice may be rubbed together until sufficient warmth is developed to _melt them both_. The axles of railway carriages frequently become _red hot_ from _friction_. 90. _Have vegetables heat?_ Yes; whenever oxygen combines with carbon to form carbonic acid gas, an extrication of heat takes place, however minute the amount. Such a combination occurs much more extensively during the germination of seeds and the impregnation of flowers, than at any other time. In the germination of barley heaped in rooms, previous to being converted into malt, it is well known that a _considerable amount of heat is developed_. 91. _Has any investigation of this subject ever been carefully made?_ Yes. Lamarck, Senebier, and De Candolle, found the flowers of the _Arum Maculatum_, between three and seven o'clock in the afternoon, as much as 7 deg. Reaum. warmer than the external air. Schultz found a difference of 4 deg. to 5 deg. between the heat of the spathe of the _Canadian pinnatifolium_ and the surrounding air, at six to seven o'clock p.m. Other observations have established differences of as much as 30 deg. between the temperature of the spathe of the _Arum cordifolium_, and that of the surrounding atmosphere.