362 



MODES OF TESTING BUILDING MATERIALS. 



[1855. 



pulverised marine salt. The whole is mixed in a porcelain capsule, 

 heated to about 200°. The combination will soon take place, with dis- 

 engagement of heat. The liquid thus obtained is to be introduced into 

 a capsule of glazed porcelain, which is maintained at a temperature of 

 about 200°. The negative electrode is a plate of platinum, upon which 

 the aluminium will be deposited, mixed with marine salt, in the form 

 of a greyish layer. The positive electrode consists of porous vessels, 

 perfectly dry, and containing melted chloride of aluminium and sodium, 

 in which is immersed a cylinder of charcoal, which generates the 

 electricity, and to which pass the chlorine, and a small quantity of 

 chloride of aluminium, arising from the decomposition of the double 

 salt. The double fixed chloride is re-constituted, and the vapours 

 cease. A small number of elements are necessary for decomposing the 

 double chloride, which presents but slight resistance to the action of 

 electricity. 



"When the platinum plate is sufficiently charged with metalliferous 

 deposit, it is removed, and allowed to cool; the saline mass is then 

 cleaned off, and the plate again introduced into the current. The 

 matter thus detached from the electrode is melted in a porcelain 

 crucible, which is enclosed in an earthenware one ; and after cooling, 

 it is treated with water, which dissolves a large quantity of marine 

 salt ; and a grey metallic powder is obtained, which is, by several suc- 

 cessive meltings, foi-med into a single mass; the double chloride of 

 aluminium and sodium being employed as a flux for that purpose. 



The first portions of metal obtained by this process arc nearly always 

 brittle ; as tine a product may, however, be obtained by it as by the 

 sodium process ; but the chloride of aluminium employed for that 

 purpose must be purer. In fact, by the sodium process, the silicium, 

 sulphur, and iron are carried off by means of hydrogen, — the iron 

 passing off in the state of protoohloride ; whilst all these impurities 

 remain in the liquid which is decomposed by the battery, and are 

 carried off along with the first portions of metal reduced. 



In addition to these processes of M. Deville, we have 



M. Bunsen's Method of Peeparation. — Taice oxide of aluminium 

 obtained either by the calcination of ammoniacal alum, or from sul- 

 phate of alumina, or by the decomposition of alum by chloride of 

 barium ; and having mixed it with charcoal, introduce the mixture into 

 a stone retort capable of containing about two quarts, and cover it with 

 a thick layer of cement composed of argil and iron scales. Place the 

 retort in a reverberatory furnace, with its neck projecting horizontally 

 therefrom, from 3 to 5 inches, and connect this neck with a glass 

 receiver, for the reception of the chloride of aluminium, which is sub- 

 limed on the introduction of chlorine. This gas is introduced into the 

 glass receiver -by a tube of large diameter, made of glass not easily 

 fusible. The stone retort is heated to a dull red heat, and a current of 

 chlorine (well washed and dried) is caused to pass therein. Chloride 

 of aluminium is then freely formed ; and at the expiration of some 

 hours the receiver will at least contain half a pound of product. When 

 this chloride is well cooled, it is mixed with its equivalent of melted 

 and pulverised chloride of sodium, and heat is applied thereto. The 

 mixture will melt at a temperature below 200° centigrade. It is 

 introduced into a closed porcelain crucible, divided into two compart- 

 ments by a porcelain partition which does not quite reach to the bot- 

 tom, and closed by means of a porcelain cover, having two holes for 

 the reception of the conductors of the battery. Six or eight pairs of 

 Bunsen's plates will suffice to separate the aluminium. If the temper- 

 ature remains at 200° centigrade, the metal will be deposited in the 

 state of powder ; and, for the purpose of converting this into a com- 

 pact mass, pulverised chloride of sodium is gradually inti'oduced into 

 the mixture, until the liquid has reached the temperature of the melting 

 point of silver. After cooling, large balls of aluminium will be found 

 in the mass, which are caused to unite by throwing them into melted 

 sea salt. The ingots thus obtained possess all the characteristics of 

 M. Deville's aluminium. 



l>Iodes of testing BiiUdiiig Mateiials* 



At the last meeting of the American Association for the advancement 

 of science Prof. Henry read a paper on the modes of testing building 

 materials and an account of the marbles used at Washington. Hehadbeen 

 appointed on a committee to test the material offered for the extension 

 of the Capitol at Washington. The committee had to take into con- 

 sideration many minute sources of disintegration, such as that every 

 flash of lightning produced an appreciable amount of nitric acid, 

 which diffused in rain water acted on the carbonate of lime, and the 



action of dust carried by the wind against the building. The com- 

 mittee subjected specimens to actual freezing and after several experi- 

 ments a good method was obtained. It was found that in ten thousand 

 3'ears one inch would be worn from the blocks by the action of frost. 

 Blocks of 1 J inch cube were subjected to pressure, and thin plates of 

 lead, as had been the case in former experiments, being introduced to 

 equalize any equalities which might occur in the surface. But upon 

 experiment it was found that while one of these cubes would sustain 

 60,000 pounds without the lead plates, it would sustain only 30,000 

 with them. They had therefore to invent a machine to cut the sides 

 of the block perfectly parallel, when it was found that the marble 

 which was chosen for the Capitol, from a quarry in Lee, Massachu- 

 setts, would sustain about 25,000 pounds to the square inch. The 

 manner of its breaking was peculiar. With the lead plate interposed, 

 the sides which were free first gave way, leaving the pressure on two 

 cones whose bases joined the plates, and whose apexes met each other, 

 and that they then yielded with comparative ease. This marble 

 absorbed water by capillary attraction, and in common with other 

 marbles was permeable to gases. Soon after the workmen commenced 

 placing it in the walls it exhibited a brownish discoloration although 

 no trace of it appeared while the blocks remained in the stonecutter's 

 yard. A variety of experiments were made with a view to ascertain 

 the cause of this phenomenon, and it was finally concluded to be due to 

 the previous absorption by the marble of water holding in solution 

 organic matter, together with the absorption of another portion of 

 water from the mortar. To illustrate the process, he supposed a fine 

 capillary tube with its lower end immersed in water, whose internal 

 diameter was sufBciently small to allow the liquid to rise to the top to 

 be exposed to the atmosphere. Evaporation would take place at 

 the upper surface of the column, and new portions of water would 

 be drawn up to supply the loss, and if this process were continued any 

 material which might be contained in the water would be found 

 deposited at the top of the tube, the point of evaporation. If, how- 

 ever, the lower portion of the tube were not furnished with a supply of 

 water, the evaporation at the top would not take place, and the depo- 

 sition of foreign matter would not be exhibited, even though the tube 

 itself were filled with water impregnated with impurities. The pores 

 of the marble, so long as the blocks remained in the yard, were in this 

 last condition, but when the same blocks were placed in the wall of the 

 building the water absorbed from the mortar at the interior surface gives 

 the supply of liquid necessary to carry the coloring materials to the ex- 

 terior surface and deposit it there at the mouths of the pores. The cause 

 of the phenomenon being known, a remedy was readily suggested ; the 

 interior surface of the stone was coated with asphaltum, rendering it 

 impervious to the moisture of the mortar, and the discoloration was 

 gradually disappearing. In a series of experiments made some ten 

 years ago he had shown that the attraction of the particles for each 

 other of a substance in a liquid form was as great as that of the same 

 substance in a solid form. Consequently, the distinction between 

 liquidity and solidity did not consist in a difference in the attractive 

 power occasioned directly by the repulsion of heat ; but it depended 

 upon the perfect mobility of the atoms, or a lateral cohesion. He 

 might explain this by assuming an incipient crystallization of atoms 

 into molecules, and consider the first effect of heat as that of breaking 

 down these crystals and permitting each atom to move freely around 

 every other. When this crystalline arrangement was perfect, and no 

 lateral motion allowed in the atoms, the body might be denominated 

 perfectly rigid. We had approximately an example of this in cast 

 steel, in which no slipping took place of the parts on each other, or no 

 material elongation of the mass ; and when a rupture was produced by 

 a tensile force, a rod of this material was broken with a tranverse 

 fracture of the same size as that of the original section of the bar. In 

 this case every atom was separated at once from the other, and the 

 breaking weight might be considered as a measure of the attraction of 

 cohesion of the atoms of the metal. The effect, however, was quite 

 different when we attempted to pull apart a rod of lead. The atoms 

 or molecules slipped upon each other. The rod was increased in 

 length and diminished in thickness until a separation was produced. 

 Instead of lead we might use still softer materials, such as wax and 

 putty, until we arrived at a substance in a liquid form. This would 

 stand at the extremity of the scale, and between extreme rigidity on 

 the one hand and extreme liquidity on the other, we might find a 

 series of substances gradually shading from one extremity to another. 

 According to the views he had presented, the difference in tenacity of 

 steel and lead did not consist in the attractive cohesion of the atoms, 

 but in their capability of slipping upon each other. From this view it 

 followed that the form of the material ought to have some effect upon 



