138 HUXLEY MEMORIAL LECTURES 



In certain minerals, notably the brown variety 

 of mica known as biotite, the microscope reveals 

 minute circular marks occurring here and there, 

 quite irregularly. The most usual appearance is 

 that of a circular area darker in colour than the 

 surrounding mineral. The radii of these little 

 disc-shaped marks when well defined are found 

 to be remarkably uniform, in some cases four- 

 hundredths of a millimetre and in others three- 

 hundredths, about. These are the measurements 

 in biotite. In other minerals the measurements 

 are not quite the same as in biotite. Such minute 

 objects are quite invisible to the naked eye. In 

 some rocks they are very abundant, indeed they 

 may be crowded together in such numbers as to 

 darken the colour of the mineral containing them. 

 They have long been a mystery to petrologists. 



Close examination shows that there is always 

 a small speck of a foreign body at the centre of 

 the circle, and it is often possible to identify the 

 nature of this central substance, small though it 

 be. Most generally it is found to be the mineral 

 zircon. Now this mineral was shown by Strutt 

 to contain radium in quantities much exceeding 

 those found in ordinary rock substances. Some 

 other mineral may occasionally form the nucleus, 

 but we never find any which is not known to be 

 specially likely to contain a radioactive substance. 

 Another circumstance we notice. The smaller 

 this central nucleus the more perfect in form is 

 the darkened circular area surrounding it. When 



