176 HOW TO WORK 



often form a good general opinion on the intensity, and may thus 

 distinguish different minerals whose intensity of action varies con- 

 siderably. But besides this, the intensity, but not the character of 

 the depolarised light varies according to the position of the crystal in 

 relation to the plane of polarisation of the light. There are two 

 axes at right angles to each other, and when either of them is parallel 

 to the plane of polarisation, the crystal has no depolarising action, 

 and if the polarising and analysing prism are crossed, it looks 

 black. On rotating either the crystal or the plane of polarisa- 

 tion, the intensity of depolarising action gradually increases, until the 

 axes are inclined to 45, and then gradually diminishes till the other 

 axis is in the plane of polarisation. If, therefore, we are examining 

 any transparent body, and find that this takes place uniformly over 

 the whole, we know that the whole has one simple crystalline struc- 

 ture ; whereas, if it appears as it were to break up into detached 

 parts, each of which changes independently, we know that it is made 

 up of a number of separate crystalline portions, either related as 

 twins, or quite independent of each other, as other facts may indicate. 

 By using a plate of selenite of suitable thickness, we may also ascer- 

 tain in what directions the crystal raises and depresses the tint of 

 colour given by the selenite, and can thus determine the position of 

 the principal axis of the crystal. 



As an excellent illustration of the use of these principles, I may 

 refer to the structure f pseudomorphs. We may often see in sections 

 of rocks crystals which are much broken up either by mechanical 

 violence or by incipient decomposition, and it might often be 

 extremely difficult or impossible to distinguish them from other cases 

 where the external form is also that of a perfect crystal, and yet 

 the material completely changed. In the former case polarised 

 light will often show at once that all the different portions are in the 

 same crystalline position, and related to the external form, but in 

 the latter are arranged promiscuously, independent of the external 

 form, or related to it as products of an alteration which extended 

 inwardly from the outer surface or from irregular cracks. Occasion- 

 ally most important theoretical conclusions depend on such a 

 structure, and it may be almost conclusive proof of the metamorphism 

 of masses of rock when other evidence almost fails. 



Then again we must examine and bear in mind any definite order 

 that may be found to occur in the arrangement of a number of 

 crystals, since that may indicate important differences. This depends 

 on the fact that crystals have a tendency to form with particular faces 

 perpendicular or parallel to the surface on which they grow, depend- 

 ing partly on the nature of the substance, and partly on the secondary 



