Page:Scientific Memoirs, Vol. 1 (1837).djvu/82

70 submitted to the rays emerging from £uiy one of our live screens, we shall always find the same ratios between the different terms of each series.

As to the black and the green glasses, their changes of transmission occur sometimes similar, sometimes contrary to those of the other plates. We should not however be surprised at these irregularities, as the green and black colours alter the natural diathermancy of the glass and give it an aptitude to transmit quantities of heat which will be more or less considerable in proportion as the rays issuing from the different screens possess themselves a diathermancy more or less analogous to that introduced into the vitreous substance by these two colouring materials In a note to the preceding Memoir (page 8) I have said that, for the study of calorific radiations the thermomultiplier is preferable to every former thermoscopic apparatus. The great number of experiments that I have since performed by means of that instrument have produced in my mind a thorough conviction of the truth of that opinion. As there are still many experimental researches to be made not only in that class of phænomena, of the history of which we have scarcely given an outline, but in every branch of the study of radiant heat, it is to be wished, for the interests of science, that every investigator would furnish himself with a thermomultiplier. This apparatus, in the state of perfection necessary to ensure good observations, is unfortunately one of those which a person cannot construct for himself until he has made several attempts which are attended with a great loss of time, and which cannot succeed in many places for want of the requisite means. For these reasons I have thought it advisable to put some one in Paris in the way of supplying them to the public. There are excellent ones to be had at M. F. Gourjon's, rue des Nonandières, N° 2. The description of the ingenious means employed by this able mechanic to give to the instrument every improvement which 1 was desirous of having introduced into it would occupy too much time. I shall therefore confine myself to the mention of the principal defects found in the first instruments of this kind presented to the Academy of Sciences by M. Nobili and myself (at the sitting of the 5th of September 1831), but now laid aside for the improved thermomultipliers constructed by M. Gourjon.

In the first place the volume of the thermoelectric pile was too bulky, (being from 36 to 40 centimetres square in section,) a circumstance which rendered it impossible to operate on small pencils of calorific rays: in the next place the galvanometer did not mark fractions lower than half a degree, and the magnetic needles, instead of standing at the zero of the scale, settled sometimes to the right and sometimes to the left at a particular distance for each galvanometer, amounting in some instances to 10 degrees. In fine, the mountings being almost all of wood the pieces became warped by the hygrometrical variations in the atmosphere, and the instrument was rendered unserviceable.

The thermomultipliers of M. Gourjon have thermoelectric piles the acting surfaces of which are not larger than the section of a common thermometer (3 centimetres square). As to the galvanometers they are mounted entirely in copper with the exception of the small pieces necessary for the purpose of isolation: the minuteness of their indications extends to a fourth and even a sixth part of a degree, and the needles, when at rest, stand exactly at the zero of the scale. It is almost needless to add that with these improvements the instrument has lost nothing in sensibility. .