74. Now, it must be considered that these coats are highly consolidated; that they are of very pure siliceous matter, and are utterly impervious to every substance which we know of, except light and heat. It is plain, therefore, that whatever at any time, during the progress of consolidation, was contained within the coats already formed, must have remained there as long as the agate was entire, without the least possibility of escape. But nothing is found within the coats of the agate save its own substance; therefore, no extraneous substance, that is to say no solvent, was ever included within them. The fluidity of the agate was therefore simple, and unassisted by any menstruum. In this argument, nothing appears to me wanting, that is necessary to the perfection of a physical, I had almost said of a mathematical, demonstration. It seems, indeed, to be impossible that the igneous origin of fossils could be recorded in plainer language, than by the phenomenon which has just been described. 75. The examination of particular specimens of agates and chalcedonies, affords many more arguments of the same kind, which Dr Hutton used to deduce with an acuteness and vivacity, which his friends have often listened to with great admiration and delight.[22] These, however, must be passed over at present; and I have only further to remark, that a series of the most interesting experiments, instituted by Sir JAMES HALL, and published in the Transactions of the Royal Society of Edinburgh,[23] has removed the only remaining objection that could be urged against the igneous origin of whinstone. This objection is founded on the common observation, that when a piece of whinstone or basaltes is actually melted in a crucible, on cooling, it becomes glass, and loses its original character entirely; and from thence it was concluded, that this character had not been originally produced by fusion. The experiments above mentioned, however, have shown, in the most satisfactory manner, that melted whin, by _regulated_ or by slow cooling, is prevented from assuming the appearance of glass, and becomes a stony substance, hardly to be distinguished from whinstone or lava. [Footnote 22: NOTE XVI.] [Footnote 23: vol. v. p. 43.] The experiments of another ingenious chemist, Dr KENNEDY, have shown, that whinstone contains mineral alkali, by which, of course, its fusion must have been assisted.[24] Dr Hutton used to ascribe its fusibility, in a great measure at least, to the quantity of iron contained in it: both these causes have no doubt united to render it more easily melted than the ordinary materials of the strata. [Footnote 24: Trans. R. S. Edin. vol. v. p. 85.]