Vol. XXIII. No. i3.] 



POPULAR SCIENCE NEWS. 



179 



Practical Cljenjistry aijd tlje ^rts. 



AN INTERESTING CHEMICAL 

 FAMILY. 



There is a group of three elements, two of 

 which are abundant everywhere, which are 

 of much scientific and practical interest. 

 Carbon, silicon, and boron all closely resem- 

 ble each other, and the first two are every- 

 where present, carbon forming the base of all 

 organic bodies, and silicon of most of the 

 inorganic mineral substances. 



Carbon occurs in three different forms, or 

 allotropic modifications, — the diamond and 

 graphite, which are crystalline, and a soft, 

 amorphous form, which is familiar to us in 

 the shape of charcoal, anthracite, or lamp- 

 black. All of these are the same chemical 

 substance, and, when burnt, form the same 

 carbonic dioxide ; but their physical appear- 

 ances and properties are, as we all know, 

 very different. The diamond, for instance, is 

 the hardest mineral substance known, and 

 graphite (or black-lead) the softest. Char- 

 coal is readily inflammable, while the diamond 

 only burns with difficulty in an atmosphere 

 of pure oxygen. The actual cause of these 

 modifications is, as yet, unknown. 



In its combinations with other elements. 

 — particularly hydrogen, oxygen, and nitro- 

 gen, — carbon forms an infinite variety of 

 substances, which are more commonly found 

 in organized living bodies, although many of 

 them have lately been formed by direct com- 

 bination without the aid of the mysterious 

 vital force. Every plant, every animal, and 

 even our own bodies, are made up principally 

 of the difl'^rent combinations of this element 

 with hydrogen, oxygen, a little nitrogen, and 

 traces of other elements, and the number of 

 similar compounds which the chemist recog- 

 nizes as theoretically or practically possible, 

 would mount up into the millions, although, 

 of course, only a comparatively few have 

 been actually produced. Another curious 

 fact is, that while carbon itself cannot be 

 either vaporized or even melted, its most 

 common combination, carbonic dioxide, is a 

 gas- which can only be liquefied by immense 

 pressure and intense cold. 



Silicon is the typical element of the mineral 

 inorganic world, as carbon is of organic living 

 bodies. Its most common form is silica, or 

 silicic acid (SiO^), corresponding to carbonic 

 acid (CO2). Common white sand is almost 

 entirely composed of this substance, while in 

 its pure crystallized form of quartz, or rock 

 crystal, it forms many of our most beautiful 

 minerals and gems. Ordinary silica is quite 

 insoluble in all acids except hydrofluoric, but, 

 strangely enough, there is a peculiar modifi- 

 cation which is soluble even in water, and is 

 found in many liot springs, from which it is 

 precipitated on exposure to the air, forming 

 masses of silicious sinter, as it is called, 



which sometimes cover many acres of 

 ground. 



The element silicon itself occurs in three 

 allotropic modifications, — the amorphous, 

 corresponding to charcoal, which is a brown, 

 readily inflammable powder ; the graphitoid, 

 which resembles graphite, and is incombus- 

 tible and not acted upon by hydrofluoric acid ; 

 and the crystalline form, which, although of 

 a metallic lustre, is, like the diamond, ex- 

 tremely hard, and crystallizes in octahedral 

 forms. 



Ai\ interesting point of difference between 

 silicon and carbon- is, that while the latter 

 forms innumerable compounds with hydro- 

 gen, only one compound of silicon and 

 hydrogen (SiH4) is known, corresponding to 

 marsh gas (CII4). This is so unstable that it 

 takes fire spontaneous-ly in the air, burning 

 with a brilliant flame, and giving ofl' clouds 

 of silica. A few compounds are also known 

 which are, aj^parently, similar to organic com- 

 pounds, in which the carbon is replaced by 

 silicon. On the other hand, silicon in certain 

 chemical relations seems to be allied to tin 

 and titanium, thus connecting it with the 

 metallic group. 



Boron is only found in mineral substances. 

 It usuall3' occurs as borax, or sodic borate 

 (Na.! B4 O7 10 Aq.), and is found in India and 

 in the bed of a dried-up lake in the Sierra 

 Nevadas. A large quantity is produced in 

 the volcanic districts of Italy, in the form of 

 boric acid, where it is obtained from jets of 

 steam issuing from the earth which are im- 

 pregnated witii the acid. 



The amorphous variety of boron is a brown 

 powder resembling silicon ; no graphitoid 

 variety is known, but there is a crystalline 

 form which resembles the diamond in shape 

 and lustre, and is so hard that it will scratch 

 the ruby, and even wear away the surface of 

 tiie diamond itself. This form of boron has 

 not yet been obtained in a perfectly pure 

 state, but usually contains carbon and alu- 

 minium in combination. 



The most striking characteristics of this 

 group of elements are their occurring in allo- 

 tropic forms, their refractory nature, — being 

 incapable of being vaporized, — and their 

 ability to form feeble acids bv direct union 

 with oxygen. The universal distribution of 

 the first two members, one in the organic, and 

 the other in the inorganic world, is a very 

 curious circumstance ; and, altogether, the 

 "carbon group" of elements is well worthy 

 of still further study by chemists. 



THE EXPANSION OF WROUGHT- 

 IRON. 



It is a general law, with but very few 

 exceptions, that all substances expand, or 

 increase in size, as their temperature increases. 

 For a few degrees above the freezing point, 

 water contracts instead of expanding, but. 



after the critical point — 39.2° F. — is reached, 

 it expands like any other liquid. Cast-iron, 

 bismuth, and antimony also contract on pass- 

 ing into the liquid form. 



The amount of this expansion varies with 

 different substances, and, although it is very 

 small, it is a very important matter, as the 

 force is almost irresistible. Wrought-iron is 

 now so extensively used for bridges, buildings, 

 machinery, etc., that, unless proper arrange- 

 ments are made to allow the free movement 

 of their parts to a certain extent, serious results 

 might ensue. The rails of a railroad, for 

 instance, are never placed close together, but 

 a small space is left between them, to allow 

 for their varying length under different condi- 

 tions of temperature. If this precaution is 

 neglected, the rails will bend, or "spread," 

 and trains have been thrown from the track 

 from this cause. 



The amount of this enlargement — that is, 

 the coefficient of expansion — of wrought- 

 iron is 0.000012204 for every degree of tem- 

 perature (centigrade.) That is, a bar of 

 iron measuring one foot at 0° C, will measure 

 1. 000012204 feet at 1° C, and this increase is 

 practically constant up to the boiling-point of 

 water. 



Knowing the coefficient of expansion, it is 

 an easy matter to construct a formula for the 

 determination of the exp«nsion of any piece 

 of iron. It is l' = i(i+a i), where / equals 

 the length at the given temperature, /' that at 

 the required temperature, a the coefficient of 

 expansion, and t the difference in centigrade 

 degrees between the two temperatures. 



Some interesting results may be obtained 

 from the use of this formula. For instance, 

 we may desire to know how much higher the 

 Eiffel Tower is on a hot day than a cold one. 

 Assuming that the temperature varies between 

 0° and 32° C, — equal to 33° and 90° F., — we 

 find that the total expansion under these vari- 

 ations is 4.7 inches, which may be taken to 

 fairly represent the difference between the 

 summer and winter height of that celebrated 

 structure. 



Applying the same calculations to lines of 

 railroad, we find that, under similar variations 

 of temperature, a line 100 miles long would 

 increase about 200 feet, while the total increase 

 of a line 3,000 miles long — say from Boston 

 to San Francisco — would be about one and 

 one eighth miles, and the total mileage of the 

 country (150,000 miles) would be nearly 60 

 miles greater at the higher temperature. 



The immense force exerted by this expan- 

 sion and contraction of iron was put, some 

 years ago, to a very ingenious practical use, 

 in restoring to a perpendicular position the 

 walls of the Paris Conservatory of Arts and 

 Trades, which had settled so as to endanger 

 the safety of the building. Iron rods, provided 

 with a screw-thread and nut at each end, were 

 passed loosely through the walls, and heated 



