Page:Scientific Memoirs, Vol. 1 (1837).djvu/54

42 being, in those different circumstances, submitted to the same quantity of radiant heat, the different degrees of diminution suffered by this heat in passing through it must evidently be attributed only to the peculiar quality of each radiation. This reflection will give still greater force to the truth of the consequences which we are about to deduce from the results of our experiments.

Seven plates of glass of different degrees of thickness submitted to the action of the four sorts of calorific rays in succession have given the following transmissions:

Although we do not exactly know the degree of heat given by the flame of oil or by platina kept in a state of incandescence by an alcohol lamp, we are nevertheless quite certain that the first of these possesses a higher temperature than the second, and that this again exceeds the 390° of the first plate of copper. Now a glance at the table is sufficient to show that the number of rays transmitted by the same plate decreases with the temperature of the calorific source, a fact which confirms the well-known law of Delaroche. But the decrease is more or less rapid in proportion to the greater or less thickness of the plate.

Let $$OM$$, $$ON$$, (Plate I. Fig. 1.) be two rectangular axes of the same length; let the first represent the thickness of the screen of 8mm and the second the total quantity of incident heat. Let us divide $$OM$$ into six parts, $$Oa$$, $$Ob$$, $$Oc$$, $$Od$$, $$Oe$$, $$Of$$, respectively equal to $$OM$$,  $$OM$$,  $$OM$$,  $$OM$$,  $$OM$$, and  $$OM$$; and through the points of division let us draw the perpendiculars $$aa' = \frac ON$$, $$b b' = \frac ON$$, $$c c' = \frac ON$$, $$d d' = \frac ON$$, $$ee' = \frac ON$$, $$f f' = \frac ON$$, $$M g' = \frac ON$$. The curve ($$a' b' c' d' e' f' g'$$) passing through the extremities of these perpendiculars will represent the decreasing intensity of the Locatelli lamp at each point of the screen of 8mm in thickness.