ADHESION. 



ADIPOC1RK. 



opposite dragging forces, the driving-wheels of the engine urging the 

 rails backward while the train tend* to urge them forward. Professor 

 Johnson hu aeen rail* entirely detached on a line where the transit 

 wai all in one direction, and where the can descended by gravity, 

 while in other cues the inclined petition of. the heads of the spikes 

 xhowed that they were prcascJ into an obliqur .. r leaning position in 

 the wooden nil. The mode of performing the experiments was to 

 drive each spike to a certain distance above its cutting edge into the 

 edge of a piece of plank or scantling, and to draw it out by a direct 

 longitudinal strain by means of a suitable apparatus. This consisted 

 of a strung bond or strap of iron connected with a weighing-beam, 

 lii.-h held the piece of plank, and a clasped pincers, with a suitable 

 jaw, for taking hold of the head and projecting part of the spike ; this 

 was attached to the opposite port of the machine, which, being 

 tightened by a strong screw, held the spike firmly while the 

 application of weight* upon the longer arm of the lever drew the 

 timber away and released the spike. Care wax taken to cause the 

 t*tnun to pans through the axis of the spike, and, by a very gradual 

 application of weighU, to avoid surpassing that force which was just 

 sufficient for iU extraction. 



The first experiments were with Burden's patent square spike* with a 

 cutting edge for driving across the grain of the timber. This spike was 

 375 inch square, and was driven into a sound plank of seasoned Jersey 

 yellow pine to the depth of 8 j inches ; the force required to extract 

 it was 2052 Ibs. ; the weight of the part driven into the wood was 

 666 grs. Troy. The second trial was upon a flanched, grooved, and 

 swelled spike, having the grooves between two projecting wings or 

 flanches on the some sides as the faces of the cutting edge ; the other 

 two sides were planes continuing to the head. This spike was driven 

 into the same kind of wood as before, and the weight required for 

 extracting it was 1596 Ibs.; the weight of the part driven in wax 

 7081 grs. The cutting edge was ragged and irregular, and the distance 

 to which it was driven was 3 j inches as in the first trial. " To know 

 the relative values of the two forms of spikes, we have but to divide 

 the weight required for the extraction of each by the number of grains. 

 in the part which had been buried in the wood. Thus, 2052-7-866 

 =2-37, and 1596---708-26=2-112. Hence, the plain spike had an 

 advantage over the swelled and grooved one in about the proportion of 

 23 to 21. It should be mentioned, also, that the plain spike was drawn 

 out by a very gradual addition of force ; whereas the force of 1596 Ibs. 

 drew the grooved spike immediately after its application. In the first 

 trial, an attempt was made to detect any yielding or gradual retreat of 

 the spike before the final start, but none was perceived." 



The third and fourth experiments were made with the same spikes 

 respectively as the first and second ; but instead of yellow pine the 

 timber employed was thoroughly seasoned white oak. The plain spike, 

 driven 3 j inches into that timber, required for its extraction a force of 

 39 10 Ibs., and, as before, exhibited no signs of movement until the 

 instant of starting, when it suddenly came out about a quarter of an 

 inch, or as far as the range of motion and the elasticity of the machine 

 would permit. The flanched, swelled, and grooved spike, driven 

 3] inches into another part of the same piece of plank, from which the 

 plain one had been extracted, was drawn out with a force of 3791 Ibs. 

 A slow motion, to the extent of A or fo of an inch, was in this trial 

 perceived to precede the starting of the spike, and was accompanied by 

 a gradual protrusion of the fibres of the timber immediately around 

 tha iron. In these experiments, though the plain spike bore the 

 greater absolute weight, yet, when the weight of metal is considered, 

 it is seen that the relative values of the two are 4 '51 5 in the plain, and 

 5'354 in the grooved form. " Hence, it appears that in yellow pine the 

 grooved and swelled form was about five per cent, less advantageous 

 than the plain, while in the seasoned oak the former waa 184 P* r <*"' 

 superior to the latter. It is also apparent that the advantage of 

 seasoned oak over seasoned yellow pine, for retaining spikes, is, by a 

 comparison of experiments 1 and 8, as 1 to 1'9 ; and by a comparison of 

 2 and 4, it is as 1 to 2'37." Other experiments were also tried upon a 

 pike of given form, which was driven a certain distance into different 

 timbers, and the order of rctentiveness beginning with the highest was 

 as follows: 1. Locust; 2. white oak; 3. hemlock; 4. unseasoned 

 chestnut ; 5. yellow pine. The results of numerous experiments are 

 given in tables, from which it appears, among other conclusions, that 

 the fibres of the wood should press the faces of the spike as nearly as 

 possible in their longitudinal direction, and with equal intensities, 

 throughout the whole length of the spike. The following arrangement 

 gives the spikes in the order of their ratios of retention to weight : 



1. HMTOW flit 7-049 



J. WUtemt S-71J 



. Grooved, but not iweUed S-663 



4. Orourrd ind notched S-SOO 



4. Oroorrd and iwtUed 4-024 



I. Burdon'. p.lcnt 4-309 



7. Sqnirr huramernl plkc 4-129 



(. Plain cylindrical 3200 



In promoting the adhesion of surfaces, it is not unusual to make one 

 of the solids semi-fluid or ductile by heat or otherwise, and on returning 

 to it* hard state, provided it does not crystallise, the adhesion is not 

 diminished unless indeed the unequal contractions of the two substance* 



by the same reduction of temperature forces them apart. The fitues* 

 of different cements for joining different substances depends chiefly on 

 their expanding equally with those substances. Metals, being generally 

 more expansible than other solids, will not adhere to ordinary cements, 

 but require metallic ones, or nJJrrt, as they are called, and these must 

 be so compounded at to have nearly the same rate of expansion as the 

 metals which they are intended to unite. " I! lass and stones, being 

 both lens expansible and less various in their rates of expansion, adhere 

 to a greater variety of substances ; and light porous solids, being very 

 little affected by change of temperature, adhere still more generally to 

 those of the same kind, which may be softened by heat or liquid 

 solvents, and return to the solid state gradually and without tendency 

 to crystalline. The most universally adhesive bodies (as pitch) are 

 those which retain, even at low temperatures, n certain ductility by 

 which they may readily yield to the various expansions and contractions 

 of the rigid bodies in contact with them ; and the rigid bodies most 

 readily made to adhere are the worst conductors of heat, because they 

 cannot undergo very sudden changes of tempc-r.it un- or bulk, and so 

 allow time for the adhering body to accommodate itself. The 

 adhesiveness of animal membranes is very remarkable, and probably a 

 provision for their cleanliness and freedom from foreign matter. 

 Crystallisation, by requiring a body while solidifying to oliey with 

 rigour certain internal laws of its own, is a great bar to adhesion. The 

 adhesion of two solids is generally stronger than the cohesion of at 

 least the weaker one. Hence, if two pieces of wood be glued, gummed. 

 or pasted together, and then separated, a layer of cement . a< Hi 

 each ; and if there be a bank-note in the midst of the cement, it will 

 be split into two layers, simply because its own cohesion is less than 

 that of the cement, or of the wood, or the adhesion between the cement 

 and either wood or paper." * 



Adhesion between solids is the chief cause of that resistance to 

 motion which is termed /rirfion. Friction ia generally greater between 

 similar kinds of matter than between different kinds. Thus, an iron 

 axle experiences more friction when moving in an iron socket than in 

 a brass one, and in order to reduce the amount of friction in machmen . 

 it is usual to interpose between the grinding surfaces some such 

 lubricant as plumbago, or grease, the particles of which have but little- 

 tendency to cohere. 



The adhesion of caoutchouc fits it for a variety of application*. 1;- 

 romplete adhesion to glass makes it useful for stopples, and enable* tin- 

 chemist to make air-tight and flexible joints in his apparatus. It i* 

 also well adapted, by its adhesion, to be employed for bonds for driving 

 machinery. Its adhesion is ingeniously employed in the manufacture 

 of i-urili. used for carding cotton. A cotton-card consists of a wire- 

 brush with a flexible bock, the latter being supplied at one time by 

 leather ; but as the holes through which the wire teeth were passed, 

 became enlarged by use, the teeth were no longer held in their places ; 

 but by making the flexible back of two pieces of linen with an inter- 

 posed layer of caoutchouc, the adhesion of the latter to the wire caused 

 it to follow the teeth when any strain wax put upon them, and by its 

 elasticity restored each tooth to its proper position when the strain w.is 

 removed. 



ADIPIC ACID (2HO, C,,H.O.). When ..lei,- ... id. *peim.i.-i-ti, fat, 

 or wax is acted on by nitric acid, several new acids are formed, and 

 amongst them adipic acid. It crystallise* in hemispherical, radiated 

 tufts, which fuse at -Jiiii , and distils without decomposition. It is very 

 soluble in hot alcohol or ether, and also in boiling v. 



ADIPIC ETHER (C..H.O, + 2C.H.O), the compound of adipic 

 acid with oxide of ethyl. It is an oily body, possessing a powerful 

 odour of rennet apples. Its sp. g. is 1-001, and it boils at 446. 



ADIT, pliuiso.) 



ADIPOCIKE, a substance so named from mltp*, fat, and ctra, wax, 

 because it possesses the properties partly of fat and partly of wax. It is 

 a body of a peculiar nature, being intermediate between fat and wax, and 

 bearing a close resemblance to spermaceti. This name was given by 

 M. Fourcroy in 1786 to the substance in question, which he dis. 

 on examining a piece of human liver that had remained for ten years 

 exposed to the air in the laboratory of M. Poultier de la Salic. In the 

 same year Fourcroy had the opportunity of observing an accumulation 

 of adipocire on a scale of prodigious extent, under circumstances of a 

 peculiar nature, which are highly curious. There was in Paris an im- 

 mense burial-ground, called La Cimetidre des Innoceus. This place 

 had been the receptacle of the dead for a considerable part of the 

 population of Paris for several centuries. On account of some im- 

 provements in the neighbourhood it was determined to mno\e thN 

 cemetery. The number of burials in this place had amounted to some 

 thousands annually. The bodies were deposited in pita or ti > 

 about thirty feet deep ; each pit was capable of holding from twelve to 

 fifteen thousand bodies ; and as the piU became full they were , 

 with a few feet of earth. The extent of the whole area was about seven 

 thousand square yards, and this space became at last occupied by a 

 mass which consisted almost entirely of animal matter, rising several 

 feet alxive the general level of the soil. Scientific men were especially 

 charged by the government to direct tin- precautions requisite for 

 securing the health of the workmen in removing this immense mass of 

 putrefying animal matter ; among whom were Fourcroy and Tlioiu.-t, 



* TomUniran'i ' Crdopwdia of I'wful Art*.' tic., A r licit Aimuiox. 



