765 



HYDRAMIDES. 



HYDRAULIC PRESS. 



768 



phiuo. although this alkaline base and many other vegetable alkaloids 

 do contain oxygen. 



Most of the hydracids are gaseous, and are easily combined with 

 water, forming solutions which possess the well known and strongly 

 marked acid properties of sourness, acting upon carbonates, and red- 

 dening vegetable blue colours. They are all artificial products, except 

 hydrochloric acid, which is sometimes disengaged from volcanoes. 



The usual method of obtaining hydrochloric acid, as well as other 

 hydracids, is that of treating a compound of the radical of the acid and 

 a base with an oxaeid and water, that generally used being sulphuric 

 acid. Thus, as already mentioned, hydrochloric acid is obtained by 

 acting upon chloride of sodium with hydrated sulphuric acid ; the 

 water suffers decomposition, and its oxygen combines with sodium to 

 form soda, while its hydrogen unites with the chlorine, giving rise to 

 hydrochloric acid ; the soda combines with the sulphuric acid to form 

 sulphate of soda. This may be taken as a type of the general action. 



HYDRAMIDES. A class of organic compounds which may be 

 described as diamidei, derived from the action of ammonia upon the 

 aldehydes. Hydrobenzamide may be taken as a type of these bodies ; 

 it is formed by acting with ammonia upon oil of bitter almonds : 



Oil of bitter almonds. 



Ilydrobenzamide. 



The hydramides are neutral cystallisable substances, insoluble in 

 water, soluble in alcohol, and volatile without decomposition. 

 The following is a list of the principal hydramides : 



Salhydramide 

 Anishydramide . 

 Cinnhydramule 

 Furfuramide 



. C 49 H. it N,0. 



HYDRANZOTHIN. [CARBAMIC ACID.] 



HYDRARGETHYL. [OKGANOMETALLIC BODIES.] 



HYDRARGOBEXZAMIDE. Synonymous with Mercury bcnzaminc. 

 [BENZOIC GROUP. Henzamide.] 



HYDRATES are compounds of water : the term is however usually 

 restricted to compounds which contain water in definite proportion, 

 which water does not impart regularity of form, or in other words give 

 crystals with the body with which it so unites. Thus when water is 

 added to potash it may form with it either water of solution, water of 

 crystallisation, or water which constitutes it a hydrate. If we take a 

 solution of potash and evaporate it to a certain extent we obtain 

 crystals of potash ; heat these and the water of crystallisation is 

 expelled, but no heat whatever is strong enough to expel the whole of 

 the water, and the List remaining portions form with the potash a 

 hydrate, which is a hard substance totally devoid of crystalline form. 

 So also when water is added to lime, a portion dissolves ; crystals 

 however of water and lime are obtained with difficulty, but hydrate of 

 lime is the well known dry powder called slacked lime. 



It appears therefore, from the above statements, that water of solu- 

 tion has comparatively little affinity for the substance with which it is 

 combined ; water of crystallisation has more, but water which con- 

 stitutes the body a hydrate has the greatest affinity of all. 



The water with which substances combine often imparts colour to 

 tin in ; thus sulphate of copper when deprived of water is nearly 

 colourless, but when dissolved in water it becomes of a fine blue 

 colour. Water of crystallisation produces the same effect. So also 

 oxide of copper is of a black colour, but the hydrate of oxide of copper 

 obtained by adding potash to a solution of copper is of a beautiful 

 blue colour. 



HYDRAULIC EN'GINEKIMXG. The branch of the arts of con- 

 struction which U more especially connected with the resistance to the 

 mechanical action of water, or with the use and application of that 

 fluid to the various purposes of life, is usually known under the name 

 of " hydraulic engineering." The objects comprised under this general 

 title are not only numerous, but they are extremely complicated, and 

 of the highest importance to nations where civilisation has been long 

 and successfully developed. So complicated, indeed, are the objects 

 the hydraulic engineer has to deal with, that there is hardly a branch 

 of the applied sciences he may not from time to time be obliged to 

 enlist in his service; and a mere enumeration of the various lending 

 divisions of his art will suffice to prove the wide range of study it is 

 necessary for him to cultivate. The divisions thus referred to have 

 been, or will be, treated under their respective heads in thin 

 Cyclopsedia. 



The hydraulic engineer, in fact, has to execute the works required 

 for the construction of BRIDGES, especially in so far aw the foundations, 

 coiler-damn, &c., are concerned ; for BREAKWATERS ; CANALS ; DOCKS ; 

 DRAINAGE; EMBANKMENTS, in water; FOUNTAINS; IHRIUATION; LIGHT- 

 HOUSES; I'IKBH; SKA DEFENCES; SEWI.K.V.I . ; \\'ATEB Sirri'i.Y TO 

 TOWNS ; WATER-WHEELS ; WELLS ; or indeed for any purpose wherein 

 it may be Decenary to control or to use water in large bodies. There 

 is little scope for the exercise of taste in these matters ; they are 

 nearly all within the domain of exact science; and no doubt their con- 



tinually increasing importance in our modern societies has contributed 

 to establish the distinction which now prevails between the pursuits 

 of the architect and of the civil engineer ; for the latter comprises 

 amongst its varieties that of hydraulic engineering. 



HYDRAULIC PRESS. This powerful engine was first suggested 

 by Pascal, based upon the hydrostatic principle of fluidity, or the pro- 

 perty of transmitting pressure equally and freely in every direction by 

 which a liquid becomes in the truest sense of the term, a machine, 



The hydraulic press consists essentially of a solid plunger, p, working 

 through a water-tight collar in a small cylinder, at the bottom of which 

 is a valve c, opening upwards, and conducting by a pipe into a cistern 



of water. In the side of the small cylinder is a channel, furnished 

 with a valve o, opening upwards, and leading by a pipe K into a large 

 cylinder B, in which is inserted a ram, passing through a water-tight 

 collar n. The top of this ram is usually furnished with a plate forming 

 the bottom bed of the press, moving in, and surrounded by, suitable 

 frame-work. Now supposing both the cylinders c B with the connect- 

 ing-pipe K to be filled with water, and the plunger p to be forced 

 down from the top to the bottom of the small cylinder, the pressure 

 thus produced will be transmitted by the water, through the pipe K to 

 the ram in B, and according to the principle of fluidity [HYDROSTATICS] 

 every square inch of the section of the ram will be pressed upwards 

 with a force equal to the downward pressure on each square inch in 

 the section of the plunger p. For example ; if the diameter of the 

 section of the rani be twenty times that of the plunger, the upward 

 pressure of the ram will be 400 times the downward pressure given to 

 the plunger. Of course the pressure on the ram will be communicated 

 to the bottom bed of the press, and any object placed upon it so as to 

 be enclosed between it and the top of the frame will be compressed. 



The extent to which the ram rises in the large cylinder must depend 

 upon the relation of its diameter to that of the plunger. The plunger 

 in descending it cylinder, displaces as much water as is equal to the 

 volume of the portion that descends, and this water being driven 

 into the large cylinder B, which is already quite full, the ram must rise 

 through a height sufficient to afford room for the water which is thus 

 forced into the cylinder, and the rise of the ram will clearly be less 

 than the descent of the plunger, in the same proportion in which the 

 sectional area of the plunger is. less than that of the ram. If , as already 

 supposed, the section of the ram be 400 times that of the plunger, the 

 ram will rise through the 400th of an inch for every inch of descent 

 made by the plunger. When the plunger is drawn up the cylinder 

 preparatory to another stroke, the valve c opens, and admits a quantity 

 of water, equal to the ascent of the plunger. And to prevent the ram 

 from falling during this operation, the valve o is placed so as to inter- 

 cept the water. When the plunger is again driven down, the pressure 

 closes the valve c more firmly, and opens o. At r is a valve closed by 

 a screw, communicating with a pipe that leads to the cistern in which 

 the pump is immersed. When it is required to relieve the ram from 

 pressure, and cause the bed-plate of the press to descend, this valve is 

 opened, the ram descends, and the water from B is driven back through 

 K, and through the pipe by which r leads into the cistern. When the 

 press is used for very intense pressures, there is danger of bursting 

 some parts of the machine, so that to prevent the pressure from 

 exceeding a certain quantity, a safety valve i is provided. This valve 

 opens outwards, and is closed by a weight attached to the longer arm 

 of a lever, so that when the pressure transmitted to the ram exceeds a 

 certain quantity, the valve opens and relieves the press. 



As the pressure communicated to the ram acts with equal intensity 

 on the interior of the large cylinder B, the water which communicates 

 the pressure seeks to escape by the joint or collar through which the 

 ram passes into B. To make such a joint or collar water-tight was the 

 difficulty from the time of Pascal xintil that of Brainah, who in 1796 

 took out a patent for a water-tight collar, and thus made the hydraulic 

 press an available engine. The collar is formed of a piece of strong 

 leather in the form of a disc with a hole in the centre, and doubled 

 down at the edges so as to form a ring, with the concavity downward* 

 as shown in section n. As this collar exactly fits the ram, the water 

 when pressed upwards enters the concave part, and presses it against 

 the ram on one side and the surface of the cylinder on the other, so as 

 to produce a perfectly water-tight joint, which has the merit of being 

 more completely water-tight as the pressure is increased. 



