VI 1 



extension of the three-dimensional surface so as to include an unstable 

 condition of the substance, partially realisable and even well known 

 in the phenomena of a liquid passing its boiling point without forming 

 vapour, and in similar unstable conditions, was an advance in the 

 theory of this important question, the consequences of which are not 

 even now completely realised. The verification of Thomson's theories 

 on this subject has proved a fruitful field of experimental investiga- 

 tion for many workers. 



Another subject of great importance to which Professor James 

 Thomson devoted much thought and attention was that of safety and 

 danger in engineering structures, and the principles on which their 

 sufficiency in strength should be estimated and proved. He made 

 more than one weighty communication on this subject to engineering 

 societies ; and on his appointment at Glasgow, in 1873, he made it the 

 subject of the Latin address which it is the custom for a newly elected 

 Professor to read to the Senatus of the University of Glasgow. An 

 address in English on the same subject became his inaugural lecture 

 to the students of his class in engineering. 



When he took up the question, about 1862, he felt that ordinary 

 engineering practice as to testing of structures, boilers for example, 

 was both illogical and unsafe. He considered that the tests usually 

 applied were quite insufficient to permit of an engineer feeling 

 justified in risking the lives of men and the property of his employers 

 to the dangers of breakdown. It was then a common opinion that 

 severe testing should not be applied lest the structure should be 

 weakened by the test itself ; but Thomson denied that the test does 

 weaken the structure if the structure be good; and pointed out that 

 the real reason for not applying a proper test was, frequently, fear lest 

 the structure should be found far inferior in strength to that which 

 it was intended to have. The truth of Professor Thomson's conten- 

 tions is now admitted by the highest engineers ; and the best engineer- 

 ing practice has, happily, undergone a thorough reform in this 

 respect. 



Certain geological questions possessed much interest for James 

 Thomson. We have seen how, at an early age, he investigated the 

 parallel roads of Glen Roy ; and on many subsequent occasions he 

 examined with great care the places where he chanced to be residing, 

 and found and described glacier markings. He traced out, on more 

 than one occasion, specially interesting features of the ice action, 

 endeavouring to determine, by means of an examination of the 

 markings, details as to the motion of the ice, whether in the form 

 of glacier or in the form of icebergs taking the ground in shallow 

 waters. 



His attention was also directed to the jointed prismatic structure 

 seen at the Giant's Causeway in Ireland, and elsewhere. No satis- 



