a name given by Dr Wollaston to an instrument of his invention, which serves to illustrate the theory of heat. It is described by him in the Philosophical Transactions, 1813, p. 71. The form of this instrument may be readily conceived without a figure. It is a glass tube, bent into the form of the Greek letter Π, with a ball of glass, D and E, at the end of each leg; the horizontal part of the tube is one or two feet long, the legs are two inches in length each, and the diameter of each of the hollow glass balls is one inch. The ball D is half filled with water; if it contained more, the water in freezing would burst it. A vacuum is produced in the tube and ball E, by boiling the water in the ball D, whilst the capillary aperture in the ball E, through which the steam is issuing, is held in the flame of a lamp till the steam becomes weak enough to allow the melted glass to collapse and seal up the capillary aperture. The apparatus is now allowed to cool, when the ball D will be half full of water, and the ball E and the rest of the cavity of the apparatus, will contain transparent watery vapour. In this state of things, if the ball E be placed in a freezing mixture of salt and snow, the water in the ball D will be speedily frozen, although distant from the freezing mixture all the length of the horizontal glass tube. The cold of the freezing mixture produces this effect by condensing the aqueous vapour in the ball E, and thereby producing a vacuum in the part not occupied by the water; thus taking off the pressure from the water in D, so as to allow another portion of that water to assume the state of vapour, a change of form attended with a reduction of heat, or a production of cold. That sensible cold accompanies the passage of a liquid to the state of vapour, may be seen by swinging a thermometer at the end of a string, the bulb having been previously wetted with sulphuric ether; the rapid evaporation of the ether, accelerated by the renewal of air, causes the mercury in the thermometer to contract considerably, and to sink to the volume which it has at the freezing of water, or even lower.

The phenomenon exhibited by the cryophorus is of the same nature as that produced by means of the air-pump in Professor Leslie's experiment, where concentrated sulphuric acid, dry basaltic porphyry in a state of powder, or some other body that has a similar faculty of absorbing vapour, is placed under the receiver of an air-pump. A saucer of water is placed under the same receiver. When water alone is relieved from the pressure of the atmosphere by pumping out the air from the receiver, a portion of the water becomes vapour, but this vapour presses on the water, and prevents the disengagement of more vapour, so that, although cold is produced, it is not sufficient to freeze the water; but when water and an absorbent substance are placed in the receiver, the absorbent substance serves the purpose of taking up the vapour.