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from his own observations, that the laminae of blue and white ice 

 (and especially in those places in which the structure originates) 

 conform to the law of perpendicularity to the direction of greatest 

 pressure. So far from there being any tendency to produce ruptures 

 and fissures lying in the planes of the laminae in these positions, 

 they were the only positions entirely free from such tendency. And 

 hence it became so difficult to conceive how the laminated structure 

 could possibly originate in actual discontinuities^such as those to 

 which Professor Forbes had ascribed them, whether we suppose the 

 blue laminae to be produced by subsequent infiltration or any other 

 process. 



According to Dr. Tyndall's views, the law stated in the preceding 

 paragraph was an essential consequence of physical causes, to which 

 the production of the laminae was referred. He had shown, experi- 

 mentally, that if a piece of ordinary ice be subjected to direct pres- 

 sure, it will melt along fine lenticular laminae perpendicular to the 

 direction of the pressure. A similar process is supposed to take 

 place in the neve, or in any part of the glacier where the structural 

 laminae originate. Professor W. Thomson had offered an explana- 

 tion of this phaenomenon on thermal principles. It had been shown 

 by himself and his brother that the melting temperature of ice is 

 lowered by compression. Now if, in Dr. Tyndall's experiment, the 

 ice were a perfectly homogeneous substance, every portion would 

 be equally compressed; and if the uncompressed mass were only 

 just above the melting temperature, the whole would melt under the 

 compressing force. But no substance is perfectly homogeneous ; 

 and consequently the internal pressures in the experiment would 

 not be perfectly equable ; and those portions of the ice which were 

 subjected to the greatest pressure would melt the soonest, and pro- 

 duce the aqueous lamina? above mentioned in the experiment, or the 

 laminae of blue ice in the glacier. In the latter case the laminar 

 portions must first be supposed to melt, the air-bubbles to escape, 

 and the water subsequently to be refrozen to form the blue transpa- 

 rent laminae. 



Mr. Hopkins did not profess to maintain the entire adequacy of 

 the above explanation of the formation of the laminated structure, 

 though he could not but feel persuaded that it was founded in truth. 

 It seemed to explain very satisfactorily the conversion of neve, or opake 

 white ice, into transparent blue ice ; but he did not well understand 

 how transparent consolidated ice could be converted by pressure 

 into white opake ice. But still, at the bottom of an ice-fall, as that 

 of the glacier of the Rhone, the broken fragments were again united 

 into a continuous mass, and the laminated structure was reproduced 

 on a type entirely new, and conformable to the altered conditions of 

 the mass ; and, assuming a large portion of the ice descending the 

 fall to be in a sufficiently consolidated state, the process of recon- 

 struction must consist as much in the conversion of blue ice into 

 white, as of white ice into blue. Mr. Hopkins was not aware 

 whether this difficulty had been previously started, or, if so, what 

 answer had been made to it. His object was more especially to 



