134 report— 1878. 



The mean diameter and thickness of each rock-plate was ascertained 

 by callipers applied with the help of a magnifying glass to a standard 

 steel scale of inches divided into tenths and fiftieths of an inch. The 

 volume was thus obtained in cubic inches with a probable error which 

 on account of the small thickness (about half an inch) and its occasional 

 unevenness, together with some irregularity, occasionally, in the diameter 

 of the nearly circular plates, it would be fair to reckon at between a half 

 and one per cent. The rock-plate was then weighed in its state of natural 

 dryness in the atmosphere,* and its specific gravity in the dry state was 

 then deduced by comparison with its water- weight, at 253 grains of water 

 (at maximum density) to a cubic inch. The plate was then heated fourteen 

 or fifteen minutes in boiling water, and quickly transferred to a well- 

 jacketed calorimeter containing (inclusive of its own water-equivalent of 

 1500 grains) a charge of 15,000 grains of cold water, in which, with the 

 assistance of an agitator (enclosing the indicating thermometer), it was 

 allowed to disengage its heat. The time taken by the plates to impart 

 to the water in the calorimeter its highest temperature varied considerably, 

 from two or three minutes with rock-salt to about twenty-five minutes 

 with pit-coal, with the different plates, and a rough control of the relative 

 conductivities of the various specimens, already previously determined, 

 presented itself accordingly during these immersions. The calorimeter 

 was freshly supplied with water a little colder than the room for each 

 experiment, and it was besides so thoroughly protected by coverings of 

 cork and vulcanised india rubber from outward influence that no correction 

 for heat-loss by radiation was required to be applied. The lumps of rock- 

 salt were thinly painted with wet oil-paint, folded in tin-foil similarly 

 painted inside, and thus rendered waterproof, and were heated (as were 

 also the plates of chalk, plaster of Paris, and two plates of brick) in a 

 well-jacketed steam space until they acquired its temperature, before im- 

 mersion in the calorimeter. The experiments with brick were conducted 

 to determine the proportion of boiling hot water which porous rocks 

 imbibe by steaming, and by boiling them before plunging into cold water, 

 as the heat conveyed by this water to the calorimeter must be known and 

 deducted before assigning to the dry rock its real specific heat. 



After removal from the cold water of the calorimeter each rock speci- 

 men was weighed, and being regarded as now perfectly saturated with 

 water, the specific gravity of the rock in its thoroughly wet state was 

 thence obtained, which is given with parentheses, for the porous rocks, in 

 the table. A slight gain of weight was almost always observable, but 

 where this did not amount to one-half per cent., the small amount of 

 porosity which it denotes is regarded as without influence in distinguish- 

 ing the properties of the wet from those of the dry rock, and only single 

 values of the specific gravities and specific heats of these rocks are noted 

 in the table. It is to be remarked that the percentage gain of weight of 

 the absorbed water must, in order to afford the porosity of the rock as 

 the percentage volume of pores or cavities which it contains, be multi- 

 plied by the specific gravity of the dry rock (which is the bulkiness which 

 water has in comparison with the rock), and the result will be the 



* A sensible proportion, perhaps 10 or 12 per cent, of the possible quantity of 

 water absorbable by a porous rock, adheres to it when left to find its own condition 

 of dryness in the open air ; and no experiments on rocks artificially dried and freed 

 from this hygroscopic moisture in a water-bath have been attempted, but the "dry" 

 state recorded in the table is the nearly constant condition which the rock naturally 

 assumes in the atmosphere of a well-ventilated room. 



