Jl-ly 1, 1886.] 



♦ KNOWLEDGE ♦ 



271 



not have so definite a guide, according to whether or not 

 other and deeper sinkings than his own have been made 

 in the neighbourhood and the lower measures recorded in 

 detail. 



I may here note the origin of the geological technicality 

 " coal measures." To anyl ody who, like mvtelf, has had to 

 pay the wages of sinkei-s (i.e. the men who sink the shaft), 

 the etymologj' is very obvious, as the men receive a different 

 rate of pay per yard, according to the character of the 

 material through which they pass, whether " metal " or 

 " rock," and the fortnightly pay is regulated by the cha- 

 racter and amount of the /neasitres of the work done ; thus 

 the strata themselves come to be described as " measures." 



Some geologists have assumed that during the carboni- 

 ferous period the atmosphere contained a larger proportion 

 of carbonic acid than at present. This speculation is based 

 on the fact that the carbon, which constitutes the bulk of 

 plants, is obtained by them from the carbonic acid of the 

 atmosphere by a process of dissociation in which the leaves 

 or other green parts of the plant employ the solar radiations 

 to separate the caibon from the oxygen with which it is 

 combined in the caibonic acid. This theory is not so well 

 received now as when I attended Professor Jamieson's class 

 in Edinburgh, but the problem which it struggled to solve 



Mr. J. L Mott, in a communication to the British Asso- 

 ciation, 1877, concludes, as the result of careful calculations, 

 that the average amount of unoxidised carbon deposit in the 

 crust of the earth amounts to three millions of tons per squaie 

 mile; this would be represented by four millions of tons of coal, 

 or, in round numbers, 3 cwt. to the square foot. This is 1,000 

 times more than all the carbonic acid now existing in our 

 atmosphere is capable of producing. If the atmosphere had 

 at any period of the world's history contained yi.r of this 

 amount, animal life, such as indicated by animal fossils, 

 would be certiiinly impossible. Therefore some great reser- 

 voir of carbonic acid is demanded, far exceeding that ever 

 contained in any breathable atmosphere. Besides this, for 

 every 6 lbs. of carbon separated from the carbonic acid 

 16 lbs. of oxygen must be evolved. 



It is customary to pass over this difficulty rather lightly, 

 as L3ell does when he says that " we may imagine time to 

 have multiplied the quantity of airbon given out annually 

 by mineral springs, volcanic craters, and other sources, until 

 the component elements of any given number of coal seams 

 have been evolved from belcw, without anv variation in the 

 meantime in the constitution of the atmosphere." 



The contribution of volcanoes to the carbonic acid of the 

 atmosphere is effected chiefly by the decomposition of lime- 



still remains. It is one of gieat magnitude, and though 

 commonly evaded, is so fundamental that until it is solved 

 we must confess ourselves profoundly ignorant of the barest 

 rudiments of geological philosophy. 



To understand the magnitude of this problem a few 

 figures are necessary. In the first place, the quantity of 

 carbonic acid at pi-esent existing in the earth's atmosphere, 

 as determii ed b}' the mo.'-t careful analysis, is between 

 004 and 0-05 per cent, by volume, or j^'^^j part by weight 

 over the land. Over the sea it is much less, the mean of 

 the whole earth falling short of 57^7,7 paJ't by weight. The 

 mean pressure of the whole atmosphere is, in round 

 nuinbeis, 2,000 lbs. per square foot, and thus' we arrive 

 at 1 lb. as the total quantity of carbonic acid over every 

 square foot of the earth's surface. 1 lb. of carbonic acid 

 (carbonic dioxide) contains but tj'W of carbon, in round 

 nnmbei-s X\ ounces, or the quantity contained in 3i ounces 

 of ordinary roal. Therefore all the carbonic acid now 

 existing in our atmosphere is onlj' capable of producing 

 a film of coal covering the earth, and weighing 5i ounces 

 to the .'qunre foot. A cubic foot of ordinary coal weighs 

 from SO to 8-5 lbs. ; therefore the stratum of coal obtainable 

 by using up the whole of the carbonic acid in the earth's 

 atmosphere would be not quite tt^ of a foot, or less than 

 35— of an inch in thickness. 



stone and magnesian rocks — cai-bonates of lime and mag- 

 nesia. Pliny the elder was suffocated by the carbonic acid 

 from Vesuvius. If these are made to give off their carbonic 

 acid by simple heating, as in a limekiln, they practically 

 contribute no carbonic acid, as the alkali left behind is 

 greedy for carbonic acid, and sooner or later finds it in air 

 or water ; but when the carbocate-s are heated sutficiently 

 in contact with silicious rocks, the silicic acid combines with 

 the lime or magnesLa, taking the place of the carbonic acid, 

 which is released as gas. 



Those who describe this as a sufficient source of supply 

 of carbonic acid usually (or aUvays, so far as I can learn) 

 take no account of, and apparently do not understand, 

 another action which is exactly the opposite, an action in 

 which the carbonic acid of the air is absorbed and releases 

 .'silicic acid. This is mcie potent than the volcanic oppo.^ite, 

 though less striking, as it goes on steadily and continuously 

 — has been going on from the earliest geoloaical periods. It 

 is called IcaoUnisation, or the conversion of the .'■ilif ates of 

 potash and soda contained in the primary crystalline rocks 

 into kaolin, i.e. the clays that have been formed from their 

 disintegrated and decomposed materials. 



Dr. Sterry Hunt has carefully calculated the amount of 

 carbonic acid required for the production of a stratum of 

 .'iOO metres (1,640 feet) of kaolin or clay, or clay-rock, over 



