The fact that the manufacturers have hermetically sealed the top of the tubes fortunately assists in an investigation of the cause of the instrument's action, for it is evident at once that the contents of an impermeable, air-tight vessel of glass can neither be affected by variations in the atmospheric pressure nor moisture. That light is not the cause of its action can be easily shown, for the chemical substance varies in height in the dark just as it does in the light. Neither can its changes be due to the action of the electricity of the air, for the changes of the tube take place the same in a protected shelter as they do in the open air, and the electric potential inside of a shelter is always zero. We turn to changes of temperature for the cause of its variations, and find a most satisfactory explanation of its action. If it is winter, one can easily convince himself of this by taking the instrument into a warm room. If now the instrument be hung out of doors, the precipitate will in a little while increase greatly in quantity, and if the difference of temperature is considerable, the alcohol in the tube will be filled from top to bottom.
I have a tube on which I have divided the length from the top of the alcohol down to the bottom into ten parts, and these again into ten parts, so that I can read the height of the precipitate at any time in hundredths of the greatest height to which it is possible for it to extend. March 24, I found the instrument hanging in my room read 21 on the scale, temperature inside of room 73°, outside 27°. I placed the instrument outside, and in two minutes little stars began to form at the side and to move down toward the bottom and up to the top again in the centre of the tube. These continued to increase in size, and in 5 minutes the alcohol was about 1/4 full of them. As these increased and the circulation of the liquid became slower, they began to accumulate at the bottom and somewhat at the top, until in 20 minutes the alcohol was full from top to bottom. I brought it inside the house, and in a few minutes the precipitate began to take on a more attenuated appearance and to slowly decrease in height. In 20 minutes it read 21 again.
Readings of the height of the precipitate taken every 10 minutes were as follows: 4:55 P. M., 100; 5:05 P. M., 93; 5:15 P. M. 80; 5:25 P. M., 50; 5:35 P. M., 21. Another instrument with a scale arranged in exactly the same manner had been hanging outside for a month or two, and read 50 during the whole experiment. The instrument in my room was next put into warm water, and I found that at a temperature of about 100° ever particle of the precipitate would disappear.
* Other ingredients besides the camphor are now included. I find it stated that two parts camphor, one part nitrate of potash, and one part sal ammoniac are put into the alcohol, and a little water added.
If allowed to remain in the water while it cooled, little stars or flakes •would begin to appear in the alcohol, and when it had fallen to a temperature of 70° or 75°, the alcohol would be entirely filled with the precipitate. If, then, the indications of the instrument are to be relied on, it is easy to get up a storm on short notice.
Suppose, when an instrument has been carried out of a warm room and the precipitate fills the alcohol from top to bottom, that it is left outside, as was done on the first of January, with the instrument hanging outside referred to above. In such a case, after the instrument has been out some time, the column of precipitate will begin to shorten, even though the temperature remains the same; and it will be noticed that the bottom presents a more compact appearance. After about a week or 10 days, the precipitate will have entirely lost its feathery appearance, except, perhaps, a little at the top, and will present a compact granulated mass tilling about half the tube, if it is winter, and about one-quarter, if it is summer. If while the column of precipitate was shortening there was any considerable change in temperature, there might be some minor oscillations in the length of the column, but the whole tendency would be downward. When this state is attained, the precipitate dissolves very slowly in the alcohol, and the instrument is capable of going through considerable changes in temperature without much change in the height of the precipitate.
Thus, suppose in the morning the temperature was 40°, and by noon had risen 50°, the height of the column may show no perceptible decrease, because the dense crystals dissolve slowly, and the alcohol lacks considerable of containing as much as it could dissolve at the latter temperature; but when toward night the temperature again falls to about 40°, the liquid is nearly saturated; and, if by the next morning the temperature has fallen to say 32°, the liquid will be filled to a considerable height with light, feathery crystals which readily dissolve on a rise of temperature, or if the temperature remains stationary, settle to the bottom changed into the more compact granular crystals. To illustrate from observation - on January 13, the temperature in the room for about a week had been ranging from about 60° to 70°, and the height of the precipitate in the instrument had been almost steady at 23 on the scale, when the temperature in the room on the evening of the 13th fell to 52°, and light flakes of precipitate began to form. I knew the liquid was now supersaturated, and any further decrease in temperature would cause a decided precipitation.
On the morning of the 14th, the temperature had fallen to 43°, and the height of the column of precipitate was 32 on the scale. I observed the instrument inside the house and the one outside, three times a day during January and February, and it was always found that the height of the two columns varied in opposite directions whenever the temperature changes inside and outside were in opposite directions.