404 



KNO\M.EDGE. 



October. 1910. 



Irops 



air in conditions similar to those in wliich it ( 

 from the burette in the above experiment ? 



The answer to these questions is apjiarentK' 

 It has been tacitly assumeci h\ 

 geological writers that the 

 stalactitic deposits formed under 

 rail\va\' and other arches are 

 produced in preciseh- the same 

 manner as the stalactites and 

 stalagmites of a lime-stone 

 cavern. But is this so. at an\- 

 rate in the earlier stages ? 



Is it possible for the whole 

 of the lime — that is, for one 

 third or one fourth of the 

 whole bulk of the mortar con- 

 tained in the two to three feet 

 in thickness of brick, or stone 

 work, which usualh" compose 

 the masonr\- of a newly-built 

 raihva\' arch — to be changed 

 into carbonate of lime by the 

 time that the first signs of 

 stalactitic growth appear? ^- 

 Apparently not, for according 

 to Mr. Dibdin, in his " Lime, 

 Mortar and Cement." the 

 absorption of at niosplieric 

 carbonic acid gas b\' mortar, 

 although at first rapid, gets 

 gradually slower. " Mortars 

 even centuries okl are said tt) 

 still contain appreciable quanti- 

 ties of hj-drate of lime." This 



being so, it seems incredible that there could be an\' 

 free carbonic acid gas a\ailable for dissobing rab^ium 



.•\ Stalactitf fonnin 



li}iic-\\ ater is 



carbonate, at an\- rate in 

 conditions ; although it 

 ma\' be possible for the 

 mortar in damp railwax' 

 arches, through which 

 rain-water is constanth' 

 passing, to become satu- 

 rated with the gas more 

 rapidly. However this 

 ma\' be, the fact remains 

 that this complete trans- 

 formation is not necessar\-: 

 stalactites of carbonate of 

 calcium can be formed 

 from lime-water in contact 

 with the carbonic acid gas 

 of the air, and the general 

 similarit}' in appearance 

 between the stalactites of 

 such bridges — hollow 

 fragile tubes of appro.x- 

 imately even diameter — 

 and the artificiall}-formed 

 stalactite described above, 



mortar under ordmarx 



iMGliKE 2. 



One of the Crystalline Projections wliich clasp the drop 

 shown in Fi.uinc 1. as seen under the niicroscope. 



different causes. In the case of the railwa}' and 

 other arches, the rain-water percolating through the 

 mortar must dissolve some of the lime, the carbonic 

 acid gas in the rain water com- 

 liining at the same time with 

 another small portion of lime, 

 thus rendering it inscahible in 

 the water which has already 

 lost its carbonic acid gas: so 

 that the drops issuing from the 

 under surface of the arch must 

 consist practically of lime- 

 water. .\s shown above, lime- 

 water will form stalactites when 

 allowed to drop slowly, so that 

 stalactitic growth may start verv 

 shorth" after the brick- work 

 has been put into position — as 

 soon, that is — as the mortar in 

 it becomes saturated with rain- 

 water. 



That the explanation gi\-en 

 alxjve is the true one is sup- 

 ported b\' the marked difference 

 ill the method of growth of the 

 two kinds of stalactites, yiz., 

 those formed from bicarbonate 

 o[ lime and those formed 

 hum lime-water. Sir Archibald 

 ( ieikie. on p. 475 of volume 1 

 of the new edition of his Text 

 Book of Geology, says, in connec- 

 tion with chemical deposits: — 

 " Of these by far the most 

 aliiuidant is calcium carbonate. The wa\' in which 

 this substance is removed and re-deposited by per- 

 meating water can be instinctivelv studied in the 



formation of the familiar 

 sf a/act ites and stalagmites 

 beneath damp arches and 

 in lime-stone caves. .\s 

 each drop gathers on the 

 roof and begins to evapor- 

 ate and lose carbonic acid, 

 the excess of carbonate 

 which it can no longer 

 retain is depositeci round 

 its edges as a ring. Drop 

 succeeding drop, the 

 original ring grows into a 

 long pendant tube, which, 

 by subsequent dei)Osit 

 inside and outside, 

 becomes a solid stalk, and 

 on reaching the ifoor may 

 thicken into a massive 

 pillar. At first the cal- 

 careous substance is soft. 



IGUIUi 1. 



on a burette from which 



slowly drip|)ing. 



and. when dry, pulveru- 

 )oints to the conclusion lent, but by prolonged saturation and the internal 

 that these stalactites and those of lime-stone caverns deposit of calcite it becomes b)- degrees crystalline." 

 are due, at any rate in their initial stages, to quite It is here clearly stated that the original deposit 



