Cryophorus. The cryophorus, or frost-bearer, is a pleasing philosophical toy, sometimes known as a pulse glass. This instrument consists of two small glass globes united by a tube, one of which is partly filled with water. The end of the tube is hermetically sealed while the water is boiling, and thus it is constructed perfectly free from air. The part of the apparatus (A) unoccupied by the water, though apparently empty, is, in reality, filled with aqueous vapour, which checks evaporation from the surface of the water (B). If the pressure of this vapour be removed, by plunging the empty ball into a freezing mixture (which condenses the vapour), so rapid an evaporation takes place, that the water in B is frozen in two or three minutes. The notion that this glass can indicate the actual state of the pulse, is erroneous it only indicates the warmth of the hand, and then acts as a differential thermometer, i. e. indicating the difference between the heat of the globes A and B.

Cryophorus 47