The Optical Magic Lantern Journal (October 1893)

The Optical Magic Lantern Journal and Photographic Enlarger. 143 To Ascertain the Quantity of Gas in a Cylinder. In reply to a question pertaining to the above, a contemporary gave the following particulars in its issue of September Ist, “Attach a pressure guage to the cylinder, turn on the gas, when the index will point to the number of atmospheres of gas contained in the cylinder. Suppose the latter to be a ten-foot, divide the indicated number by ten, and the result will be the number of feet contained in the cylinder.” The foregoing instructions, we may say, are wrong; but in measuring the contents of so small a cylinder as a ten-foot one, the error is not nearly so exaggerated as it would be if the contents of a forty foot bottle were being computed. Let us see how the above instructions work out, taking the size of cylinder mentioned by our contemporary, 10ft. This 10ft. cylinder having just been charged we will find on applying the guage that 120 atmospheres will be represented on the dial; this, divided by the sum stated, viz., 10, will give us 12ft. of gasin a 10ft. cylinder. According to the same rule we will take a 40ft. cylinder which has just freshly been charged and gauge it in the same manner, the dial will again indicate 120 atmospheres, so according to the rule above we must divide the number of atmospheres by the capacity of the cylinder, and what do we find— Why, that when a 40ft. bottle is “full” that it only contains 3ft. Now dismissing all these misconceptions of our contemporary, the contents of a cylinder may be ascertained either by measuring the number of atmospheres in the cylinder and computing therefrom, or it may be ascertained by weighing the cylinder. With regard to the first method, after the number of atmospheres has been ascertained it is necessary that it be divided by some particular number, but this number must not be that of the size of the cylinder. It must be the number of times that the capacity of the cylinder is contained in 120. Thus for 6ft. cylinder it must be divided by 20. A 10ft. cylinder by 12; a 12ft. by 10: a 15ft by 8: a 20ft. by 6; and a 40ft. by 3. It is not our intention here to deal with the construction of the different forms of pressure guages upon the market, so we will pass on to the method of ascertaining the contents of an oxygen cylinder by weighing it. It is necessary that the exact weight of the cylinder be ascertained when it is fully charged ; this we will suppose to be 16lbs. and 2 ozs. After using some of the gas we will take it that it weighs lilbs. and Qozs., this enables us to ascertain that 9ozs. of the oxygen have been used. As each ounce weight of oxygen is equivalent to 0-7 of a foot, and as in this case 9ozs. have been used, we have merely to multiply 0-7 by 9 which gives 6°3 or 6,35ft. which has been taken out of the cylinder. —0:——. Why does a Condenser get Hot? Wary does a lantern condenser get hot? A little consideration of the principles at the root of the heating of condensers by radiants may be useful to these inventors who have to deal with the heating of all parts of the optical system of lanterns. . The chief factor in the heating of condensers is not light, for the invisible heat rays from luininous sources have far more heating power than those which can be seen. by the eye. When a man is warmed by the sun, the invisible rays heat him a vast deal more than do those which form light. So far as men of science know, there is no difference but that of wave length and velocity of vibration between rays of heat and rays of light; the apparent great difference is in ourselves. The nerves in the hand are sensitive to heat and not to light, whilst the nerves in the eyes are sensitive to light, and in a lesser degree to radiant heat. When any rays pass through a lens without obstruction they do not raise its temperature, but directly they are obstructed the body causing the obstruction gets hot, for energy is indestructible, and never. lost.. The obstructed rays must do work of some kind, and that work is the heating of the condenser in the case under consideration. Waves of invisible heat obey the same laws as light; they can be reflected by concave mirrors, they can ‘be refracted, and they can be brought to a focus. The two ends of the spectrum are invisible. When a spectrum is projected upon the screen, the ultra-red rays are invisible, but their presence may be experimentally made evident by moving a thermo-electric pile along the space on which they are falling, and reading off the amount of current set up by means of a reflecting galvanometer. Then plenty of heat rays are found to be falling where nothing is to be seen; there is much more heat there than in the whole of the rest of the spectrum. As the thermo-pile is moved along the visible spec