HEAT OP MOONLIGHT. 



19. It has long been an object of inquiry among philosophers 

 whether the light of the moon has any heat, but the most 

 delicate experiments and observations have failed to detect this, 

 property in it. A thermometer of extreme sensibility, called 

 a differential thermometer, was the instrument applied to this 

 inquiry. 



This instrument consists of two glass bulbs, A and B (fig. 5), 

 connected by a rectangular glass tube. In the horizontal part of 

 the tube a small quantity of coloured liquid (sulphuric acid, for 

 example) is placed. Atmospheric air is contained in the bulbs 

 and tube, separated into two parts by the liquid. The instrument 

 is so adjusted that, when the drop of liquid is at the middle of the 

 horizontal tube, the air in the bulbs has the same pressure ; and 

 having equal volumes, the quantities at each side of the liquid are 

 necessarily equal. If the bulbs be affected by different tem- 

 peratures,* the liquid will be pressed from that side at which the 



Fig. 5. 



50 40 30 20 "10 O ID 20 30 40 So 



temperature is greatest, and the extent of its departure from the 

 zero or middle is indicated by the scale. This thermometer is 

 sometimes varied in its form and arrangement, but the principle 

 remains the same. Its extreme sensitiveness, in virtue of which 

 it indicates changes of temperature too minute to be observed by 

 common thermometers, renders it extremely valuable as an instru- 

 ment of scientific research. By this instrument, changes of 

 temperature not exceeding the 6000th part of a degree are 

 rendered sensible. 



The light of the moon was collected into the focus of a concave 

 mirror of such magnitude as would have been sufficient, if exposed 

 to the sun's light, to evaporate gold or platinum. The bulb of 

 the differential thermometer was placed in its focus, so as to receive 

 upon it the concentrated rays of the moon. Yet no sensible effect 

 was produced upon the thermometer. We must therefore conclude 

 that the light of the moon does not possess the calorific property 

 in any sensible degree. 



This result will create less surprise when the comparative 



43 



