Nitrous oxide is supplied by the makers as a fluid condensed in iron bottles or cylinders, and only becomes gaseous upon being released from them. In passing from the fluid to the gaseous state, heat is of course lost, and it will be noticed that the end of the cylinder which is in use becomes rapidly crusted over with frost. Ice, moreover, forms in the small channel at the head of a cylinder, and is apt from time to time to block it. The cylinders are of various sizes and designated after the numbers of gallons of gas which they will supply: 25 is the smallest size, 50 or 100 are more usual; anything up to 500 is occasionally met with in hospital practice. Moreover the cylinders are of two types, called respectively _vertical_ (for use in the upright position), and _angle_ for use in the horizontal position (_see_ Fig. 13). [Illustration: FIG. 12.--Frame for adapting vertical cylinders to foot use.] [Illustration: FIG. 13A. FIG. 13B. Two types of N_{2}O cylinders. A. Vertical (or ordinary). B. Angle.] The cylinders are fixed in frames of various types of which examples are seen in Figs. 13 and 14. Upon each cylinder, of whatever size or type, will be found a label stating its weight when full and empty, the difference representing the weight of the contents when the bottle is full. For instance, in the case of the 50-gallon cylinder the weight of its full charge is 15 ounces. Weighing the cylinder is the only certain means which we have to estimate how much of the charge remains. The student will readily appreciate therefore that once a cylinder has been used at all there is always a risk of the supply of gas from it running out during an administration. It is for this reason that cylinders are habitually used in couples, one of which is always supposed to be quite full. To this one it is well to attach a label marked “full,” and care must be taken to replace at once a cylinder known to be empty. In this way we always have upon the frame one cylinder partly and another entirely full. [Illustration: FIG. 14.--Complete N_{2}O apparatus, showing twin cylinders, supply pipe, 2-gallon bag, 3-way tap, and face-piece.] By whatever makers the cylinder is supplied it will be found that the thread upon the outlet pipe is the same, and the metal nipple figured in Fig. 14 will fit it. To the distal end of the nipple, a rubber tube is attached which leads to a rubber bag usually of 2 gallons capacity. The remainder of the apparatus may be of several types.