December 11, 1885.] 



SCIENCE. 



511 



product due to recent weathering. A slight decay 

 of minute mica plates may be observed, but for 

 practical pui-poses it amounts to nothing. The 

 outer surfaces of the detached pieces are equally 

 fresh with the inner surfaces. The felspar is 

 scarcely more altered than when the rock was 

 lying unquarried in the hills at Assuan. The de- 

 cay is not due to chemical decomposition, but to 

 mechanical disintegration. 



In preparing the obelisk for its recent coating of 

 paraffine, the workman carefully picked off with 

 small iron tools all the loose scale and exfoliated 

 material still adhering to the solid rock. My 

 friend, jVIr. Samuel Parsons, superintendent of 

 parks, informs me that this refuse weighed 780 

 pounds, — a truly astonishing amount, when we 

 consider that for 3,400 years the obelisk withstood 

 the effects of time better in Egypt than during 

 the last five years in Central park. 



In my opinion, the process of disintegration has 

 been an extremely slow one, caused by a constant 

 expansion and contraction of the constituent 

 minerals near the surface, due to diurnal varia- 

 tions in temperature. In a- cHmate like that of 

 New York, where these diurnal changes are fre- 

 quently excessive at all times of the year, the 

 tension between the minerals would naturally tend 

 to a mechanical disintegration of the rock. Granite 

 being a poor conductor of heat, the effect of these 

 constant changes would be felt only at short dis- 

 tances below the surface, causing in time minute 

 fractures and fissures along lines of weakness. 

 Into these openings percolating waters, upon 

 freezing, would rapidly complete the work of de- 

 struction. The result would be what we now 

 find, — a scaling-off , or exfoliation, of the exposed 

 surfaces. In structure the rock of the obelisk is 

 coarse-grained ; and the minerals, being less firmly 

 held together than in many more compact varie- 

 ties, yield more readily to changes of tempera- 

 ture. 



Observation showed that decay progressed some- 

 what more rapidly upon the south and west faces 

 of the obelisk than upon the north and east. 

 Upon the south and west the direct rays of the 

 sun would act more intensely during the day, but 

 the temperature by night would be nearly the 

 same on all sides. Now, if the cause of the decay 

 was due to expansion and contraction, disintegra- 

 tion would be greatest on the surfaces exposed to 

 the severest strain (the south and west), notwith- 

 standing that the opposite sides would be those 

 most exposed to the driving cold storms. 



It seems most probable that the obelisk, during 

 its long exposure in Egypt, must have been slowly 

 undergoing change, the minerals losing some of 

 their cohesive power, and only required a lower 



temperature to hasten what had been in progress 

 for ages. 



Upon this subject the experim.ents of Professor 

 Wigner, published in the London analyst of 1878, 

 on rock from the twin obeUsk now standing on 

 the Thames embankment, are of considerable in- 

 terest. Pieces of the twin obelisk were placed at 

 Professor Wigner's disposal by Mr. W. Dixon, the 

 English engineer, who had charge of the removal 

 of the prostrate monolith from Egyjjt to London. 

 They represented portions from the under surface, 

 which had been buried in sands, and also from 

 the upper exposed surface. Both pieces were 

 found nearly free from chemical decomposition ; 

 and analysis showed but sHght oxidation of the 

 iron in the surface rock. Experiments, however, 

 upon the absorbent power of water of the two sam- 

 ples, gave widely different results. Professor Wig- 

 ner estimated that the sound rock, which had been 

 buried in sands, absorbed 7.8 grains of water per 

 square foot of surface ; and the weathered or ex- 

 posed rock, 46.1 grains per square foot of surface, 

 or nearly six times as much as the fresher rock. 

 He says, "The 46 grains from absorption per 

 square foot gives us a comparatively fair estimate 

 of the amount of water which can be retained in 

 the weathered surface, and which is ready, by its 

 expansion and freezing, to split or disintegTate that 

 surface still further." According to Lieutenant- 

 Commander Gorringe, a high authority, the Lon- 

 don obelisk, which stood at Alexandria until the 

 early part of the thirteenth century, was probably 

 thrown down by the severe earthquake which 

 visited northern Africa at that time. If we may 

 assume, as is probable, that for the greater part of 

 the 500 years the London obelisk was partially 

 buried in sands, the difference in the absorbent 

 IDOwer of water in the two specimens may be taken 

 as measure of the effect of climatic agencies in 

 Egypt during that period. 



The New York obelisk, subjected to precisely 

 similar agencies, would be in condition, after its 

 transportation to America, to disintegrate rapidly 

 when exposed to a lower temperature, and the con- 

 sequent freezing and melting of the water ab- 

 sorbed thi'ough the interstices. 



Arnold Hague. 



Washington, Dec. 3. 



THE MEETING OF THE A3IERICAX PUB- 

 LIC HEALTH ASSOCIATION. 



The American public health association began its 

 thirteenth annual meeting at Washington, Dec. 8. 



After the opening ceremonies, a letter was read 

 from the board of health of Montreal, stating that 

 the epidemic of small-pox in that city was started 

 by an imported case from Chicago, Feb. 28 of the 



