D. Forbes — Contraction of Igneous Rocks. 3 



Vtanced as to the solidity of the earth, I am at a loss to concoivo 

 why I should be here represented as " eoncerned to reduce as much 

 as possible the amount of contraction of its materials." However, 

 on the tu quoque principle, I shall endeavour to show whether 

 it may not in reality be the Rev, 0. Fisher who is most " con- 

 cerned to reduce as much as possible" the effect of the deductions 

 (obvious from my experimental results) upon his own theoretical 

 views : for, in the first place, he supposes that the iron moulds had 

 been measured when cold, or, in other words, rej:) resents the ex- 

 perimentalist as being probably so blind as to overlook so obvious 

 a source of error ; and, secondly, he talves it for granted that the 

 highly silicious slags which are referred to were, after casting, in a 

 vitreous, and not in the devitrified or stony condition. 



In reply, it may be stated, that the cast-iron moulds made use of 

 were constructed in parts, so as to be adjusted in capacity with ease, 

 either when cold or hot ; that they were measured whilst in tlie 

 hot or expanded state ; and lastly, that curiously enough, as it may 

 seem at first sight, the results obtained from casting in perfectly 

 cold moulds, measured when cold, were as near as possible identical 

 with those in the expanded hot moulds, measured when hot. This 

 is easily explained from the fact that the slag, when poured into a 

 perfectly cold mould, solidifies instantaneously on all sides in contact 

 with the iron long before the thick cast-iron mould has even had 

 time to expand, the slag thus forming a cube or rectangle of the 

 exact size of the original cold mould. Next, the highly silicious 

 slags which Mr. Fisher supposes would have contracted still further, 

 when they came into the devitrified condition, could not do so ; 

 simply because they were already in that condition, as is very 

 commonly the case with slags which contain a considerable per- 

 centage of lime, since, on cooling, such slags (as may easily be seen 

 at the Staffordshire and other blast furnaces) pass, in most instances, 

 at once from the molten to the stony or devitrified condition, and 

 show but little of the glassy surface or interior so characteristic of 

 other slags containing less lime. This fact is fully appreciated by 

 the glass manufacturers, who take especial care not to introduce too 

 much lime into their mixtures. 



The introductory pages of Mr. Fisher's paper, in the state in 

 which it now appears in the Transactions of the Cambridge Philoso- 

 phical Society, have evidently been considerably altered from what 

 they were when read at the Society (April 27th, 1868), since 

 reference is made both to Mr. Delaunay's researches — a translation 

 of which I communicated to the Geological Magazine of November, 

 1868 — as well as to my paper in the Chemical News of October 23rd, 

 of that year. For this reason also I might have expected, when it 

 eventually did appear in print, that I should not have been obliged 

 to make the above remarks, particularly as Mr. Fisher, under the 

 date of January 22, wrote me as follows : — " Will you allow me to 

 ask you, with reference to your article in the Chemical News of 

 October 23rd, whether, in estimating the contraction of the slags 

 cast in iron moulds, the expansion of the iron was allowed for. It 



