Mr. W. Ackroyd on Selective Absorption. 



425 



Heating or 

 expanding. 



Metachromatic Scale. 



Black 



Brown 



Bed 



Orange 



Yellow 



Green 



Blue 

 f White or 

 J Colourless. 



Cooling or 

 contracting. 



The scale may be illustrated by heating the following anhy- 

 drous bodies on white porcelain. Zinc oxide changes from 

 white to yellow and orange ; metaborate of copper from blue 

 to green and yellowish green ; baric chromate from yellow to 

 orange ; mercuric oxide from orange-yellow to orange, red, 

 and brown ; potassic dichromate, red to dark red ; trimanganic 

 tetroxide, reddish brown to nearly black ; and cuprous oxide 

 glass from scarlet to dark scarlet and black. 



Respecting this colour-change, we make out : — (1) that it 

 arises from increased structural absorption with elevation of 

 temperature, the more refrangible increment increasing at a 

 greater rate than the less refrangible ; and (2) that the only 

 necessary concomitant is alteration of atomic potentiality — a 

 change from the white towards the black end of the meta- 

 chromatic scale signifying atomic recession, and a change 

 from the black towards the white end, atomic approach. 



(1) In support of this we must refer to the bichromate ex- 

 periment. The change from red to dark red viewed spectro- 

 scopically gives spectra Nos. 1 and 2. 



(2) To arrive at the approximate cause, I have heated colour- 

 changing bodies in various media, as nitrogen, carbonic anhy- 

 dride, and hydrogen, the result of which has been to show that 

 the surrounding medium has nothing to do with metachroma- 

 tism. The phenomenon is exhibited by all three states of 

 matter — by solids (amorphous and crystalline), by liquids, and 

 by gases near their liquefying-points. By a process of elimi- 

 nation it may readily be seen that the phenomenon is due to 

 the reception of energy from the heating agent. Now of this 

 transferred kinetic energy a portion is converted into energy 

 of position. In the metachrome or colour-changing body we 

 have therefore — 



i. Kinetic energy, and the complementary 

 ii. Potential energy. This energy of position is acquired by 

 a. The overcoming of cohesion (molecular recession or 



