822 HYDRATES 



M. Bonnemain applied, with much success, these ingenious processes of heating by 

 the circulation of water to maintain a very equal temperature in hothouses (serres- 

 chaudes), in stoves adapted to artificial incubation, and in preserving or quickening 

 vegetation within hothouses, or outside of their walls, during seasons unpropitious to 

 horticulture. 



Since the capacity of water for heat is very great, if the mass of it in a circulation- 

 apparatus be very considerable, and tho circulation be accelerated by proper arrange- 

 ments, as by cooling the descending-tube exterior to the stove-room, we may easily 

 obtain by such means a moderately high and uniform temperature, provided the heat 

 generated in the fire-place be tolerably regular. We may easily secure this essential 

 point by the aid of the fire-regulator, an instrument invented by M. Bonnemain. 



It should be clearly understood that wood will become charred and eventually take 

 fire if kept in contact with iron pipes through which hot water circulates. This may 

 appear contrary to ordinary experience. There are, however, numerous evidences 

 which prove distinctly that it is dangerous to have wood in contact with even hot- water 

 pipes, and that it is infinitely more so to have it near to pipes through which the circu- 

 lation of heated air is carried on. 



There are several arrangements for carrying out this system of warming ; but the 

 principle is the same in all of them. Heated water ascendsfrom the boiler, circulates 

 through the pipes, giving warmth to the building in its passage, and it returns cool 

 to the boiler, to be again circulated. 



HOWIiXTE. A hydrous borate of lime. See BORACIC ACID. 



HUANO. See GUANO. 



HUlYTBOIiDTIIiITE. A silicate of alumina, lime, magnesia, soda, &c. It occurs 

 in crystals belonging to the pyramidal system, in the ejected limestones of Monte 

 Somma. A substance identical in chemical composition and crystalline form has been 

 found in slags from some of the Staffordshire blast-furnaces. 



HUIVIITE. A silicate of magnesia from Vesuvius ; known also as Chondrodite. 



HUMUS. The vegetable matter of the soil, called also Ulmin. By action of al- 

 kalis it yields humic or ulmic acid. 



HUNGARY WATER. Supposed to be named after a queen of Hungary, who 

 used it as a cosmetic : it is prepared by distilling rosemary. See EATT DE COLOGNE. 



HYACINTH. The name under which are included the transparent, bright- 

 coloured varieties of zircon. Hyacinth differs from jargoon merely in colour, which 

 is orange-red passing into poppy-red. Though not much worn at the present time, it 

 is a valuable gem, and makes a very superb ring-stone when of a bright tint and free 

 from flaws. The larger species are sometimes made into seals. Hyacinths occur in 

 the sand and alluvial deposits of certain rivers in Ceylon, also in the state of sand, 

 mingled with various other substances, in the bed of a stream at Expailly (Haute-Loire) 

 in France, as well as in basalt near the same place. It is also found in volcanic tuff 

 in Auvergne, in Bohemia, Saxony, the Tyrol, Transylvania, Greenland, in the zircon- 

 syenite of Fredericksvarn, in Norway, and in the iron mines of Arendal; also at 

 Miask in the Urals, Vesuvius, at Santa Eosa in New Grenada, at Scalpay in Harris, 

 Scotland, Egypt, the East Indies, and elsewhere. The hyacinth-red varieties of zircon 

 are sold by the inhabitants of Ceylon as inferior rubies. H. "W. B. 



Many of the stones used in jewellery under the name of hyacinth are nothing more 

 than hyacinth-coloured garnets, belonging to the variety known as cinnamon-stone or 

 essonite. This false hyacinth may be readily distinguished from the true gem by 

 difference of specific gravity; that of the cinnamon-stone being only about 3'6, whilst 

 the specific gravity of the true hyacinth rises as high as 4*7. Moreover, zircon is a 

 doubly-refracting mineral, and garnet singly-refracting : a distinction which may 

 sometimes be of value in the discrimination of cut stones. Both substances have 

 about the same degree of hardness. The red zircon loses its colour on ignition, and 

 increases in density. Some finely-coloured hyacinths have recently been found in the 

 form of rolled pebbles at Mudgee, in New South Wales. 



HYALOGRAPHY. The art of etching on glass. The process may be effected 

 either chemically, by the action of hydrofluoric acid ; or mechanically, by means of 

 the sand-blast. See GLASS. 



HYDRACIDS. A term sometimes applied to those acids which obviously contain 

 hydrogen as an essential constituent, such as hydrochloric acid. The term was 

 formerly used to distinguish these acids from another class called oxyacids, such as 

 sulphuric acid ; but as these also are now known to contain hydrogen, the old distinc- 

 tion has lost its significance. All acids, ind--.!, are hydracids. 



HYDRATES are compounds of tho oxi!"s. salts, &e., with water in definite or 

 equivalent proportions. Thus slaked lime consists of one atom of quicklime = 2S, 

 + one atom of water = 9, of which the sum is 37 on the hydrogen scale. 'The very 

 different functions performed by water in the various modes of combination it affects 



