NATURAL PHILOSOPHY. 239 



stance is wanting than when it is present ; that is, it must be lower 

 in vacuo than in a space filled with air. 



In order to ascertain whether this was the case, a glass apparatus 

 was used, in which a thermometer, observable from without, was 

 firmly fixed. It could be filled with different gases, and these could 

 be variously dilated. The upper part of this apparatus was main- 

 tained at the same temperature, namely that of boiling water, and 

 the temperature was observed which .a thermometer introduced into 

 the interior ultimately assumed. It was necessary that the space 

 surrounding the apparatus should always beat the same temperature. 

 In these experiments the temperature of the surrounding space was 

 15°. In this way the following results were obtained : — 



1. The temperature which a thermometer ultimately assumes in a space heated 

 from above, differs when this space is filled with different pases. 



2. In hydrogen the temperature is higher than in any other gas. 



3. In this gas the temperature is higher than in vacuo ; and the denser the gas 

 is, the higher is the temperature. 



4. Hence hydrogen conducts heat like metals. 



5. In all other gases the temperature is lower than in vacuo ; and the denser 

 they are, the lower is the temperature. 



6. It cannot hence be concluded that gases do not conduct heat, but only that 

 they do this in so small a degree that the action of conduction is cancelled bj 

 their diathermancy. 



7. This remarkable property of hydrogen is evinced not only when it moves 

 freely, but also when it is contained between eider down, or any loose substance 

 which hinders its motion. 



8. The great conductibility of this gas is a further confirmation of its analogy 

 with metals. 



9. Hydrogen conducts not only heat, but also electricity, better than other 



PERCEPTION OF COLOURS. 



Dr. Gladstone has read to the British Association a paper " On 

 his own Perception of Colours." The author described himself as in 

 an intermediate position between those who have a normal vision of 

 colours, and those who are termed "colour-blind." These latter are 

 usually unacquainted with the sensations of either red or green, and 

 it becomes a desideratum to have good observations on those who are 

 capable of acting somewhat as interpreters between them and those 

 who perceive every colour. By means of Chevreul's chromatic circles 

 and scales, Maxwell's colour-top, coloured beads, &c, the author 

 was able to determine the following points in respect to his own 

 vision. He sees red, in all probability, like other people, but it re- 

 quires a larger quantity of the colour to give the sensation than is 

 usually the case ; hence a purple appears to him more blue, and an 

 orange more yellow, than to the generality of observers. He is per- 

 fectly sensible of green, or rather of two distinct greens — the one 

 yellowish, the other bluish — but between them there lies a par- 

 ticular shade of green, to which his eyes are insensible as a colour. 

 This modifies his perception of many greens that approximate to what 

 is to him invisible. The shade occurs in Nature on the back of the 

 leaf of the variegated holly, and it may be produced in Maxwell's top 

 by certain combinations of the coloured disc ; the simplest being — 

 04*5 Brunswick Green (Blue Shade) + 5"5 Ultramarine = 9i Black + 6 White. 



