126 Prof. Knoblauch on the Interference of Heat. 
_ It is obvious, as Wurtz remarks, that by treating oxide of 
ethylene with compound ammonias, a great variety of artificial 
alkaloids containing oxygen may be obtained, 
Bohn* has investigated the optical relations of the tartaric 
acid prepared artificially by Liebig, by the action of nitrie acid 
on sugar of milk. He finds that they are quite identical with 
those of the ordinary tartaric acid. 
XVI. On the Interference of Heat. 
By Professor KnopLaucu ft, 
HE differences of phase observed by Prof. Knoblauch in 
the ztherial oscillations of interfermg thermic rays were 
produced in the following four different ways. 
1. Difference of phase in consequence of unequal lengths of path 
in one and the same medium. 
A beam of solar light, after being reflected by a heliostat, 
entered through a slit, from 4 to 6 millims. wide, into a dark 
room, and at about 2°3 metres from the window fell upon a glass 
grate behind which was placed an achromatic glass lens. When 
a square thermo-electric pile (whose anterior opening could be 
narrowed or widened by means of wings) was placed at about 
0:5 metre from the Jens in different parts of the interference- 
spectrum there formed, a multiplier connected with the thermo- 
electric pile showed deflections varying, according to the fine- 
ness of the grate, from 2°15 to 18°°5 when the pile entered 
the central white luminous field. The needle of the multiplier 
returned to its zero-point when the thermoscope was placed in 
one of the dark bands to the right or left of the centre. It 
became deflected again, however, to 0°°6 or 0°7 as soon as the 
instrument was moved into either of the next following coloured 
spectra. With very fine grates, the increased cold between the 
first and second spectra could be detected with certainty. 
The phenomenon was clearest with finely scratched plates of 
rock-crystal, behind which the indications of the thermo-multi- 
plier were the following :— 
2° in the central white . . 2°5 millims. broad, 
0° in the first dark band. . 9:0 millims. broad, 
1°25 in the first spectrum . 8-5 millims. broad, 
0° in the second dark band . 1°25 millims. broad, 
0°-87 in the second spectrum. 15-0 millims. broad. 
* Comptes Rendus, December 3, 1859. : : 
+ From the Monthly Reports of the RoyalAcademy of Sciences at Berlin. 
