FREEZING AND MELTING POINTS. 



FRESCO PAINTING. 



210 



cury by the absorption of caloric from a coating of ice in which the 

 bulb of a thermometer was enveloped. In the performance of this 

 experiment, the thermometer was suspended from a wire which, 

 passing through a collar of leathers at the top of the receiver, could be 

 raised and lowered at pleasure. A cup of water was placed above the 

 vessel containing the sulphuric acid; and the bulb of the thermometer, 

 after being lowered into the cup, was drawn up and suffered to lose 

 caloric by the evaporation of the adhering water, in the exhausted 

 receiver : this process being repeated several times, the bulb was at 

 length covered with a film of ice. The cup of water was then removed, 

 and the thermometer, having its bulb thus covered, was allowed to 

 remain suspended about half an hour above the sulphuric acid, the 

 exhaustion of the air being made as complete as possible, so that the 

 column of mercury in the barometer-gauge was within l-20th inch as 

 high as in a barometer which indicated the density of the external air. 

 On re admitting the air, and removing the receiver, a portion of the 

 mercury in the ball of the thermometer was found to be frozen. In 

 another experiment, the mercury was inclosed in a mass of ice 

 worked by heat into the form of a cup, and suspended above the sul- 

 phuric acid in a small net made of wire. The mercury was introduced 

 into the cup of ice, and on it was poured a small quantity of water. 

 On exhausting the receiver, the water was immediately frozen, so that 

 the mercury was then completely enveloped with ice ; and in a few 

 minutes, on removing the receiver, the mercury was found to be 

 wholly frozen. [EVAPORATION.] 



FREEZING AND MELTING POINTS, a term applied to the 

 temperatures at which certain liquids become solid, or certain solids 

 become liquid. Thus, mercury becomes solid at 39 Fahr., oil of 

 vitriol at 30*, bromine at 4*, water at 32", phosphorus at 111-5, 

 and so on. These points are also entered as fusing points, [FUSION.] 

 Thus we say that yellow wax, on the reduction of the temperature, 

 becomes solid, and also, on being heated, begins to fuse at 143'6, 

 iodine at 224'6, sulphur at 239, tin at 451, nitrate of soda at 

 591, lead at 620, nitrate of potash at 642, zinc at 773", antimony at 

 about 900, silver at 1773, copper at 1996, gold at 2016, cast iron at 

 2786, and wrought iron at about 3280. 



Although there is nothing essentially different in the passage of 

 water and cast iron from the fluid to the solid state, yet for tempe- 

 ratures above that at which water becomes solid, the term freezin;/ is 

 not usually applied, but rather the point of ivlidiji'-.ation, or, as before 

 stated, the fusing point. It is remarkable that the freezing point of 

 water, a liquid which expands in freezing, becomes lowered to a minute 

 but measurable extent by exposing the water to strong pressure. 

 According to Professor Thomson, of Glasgow, a pressure of 8'1 atmo- 

 spheres lowers the freezing point 0'106 Fahr., and a pressure of 16'8 

 atmospheres 0'232; whereas, in bodies which contract in becoming 

 solid, the melting point is found to be raised by pressure. Thus, 

 spermaceti, which under atmospheric pressure became solid at 117'9, 

 under a pressure of 150 atmospheres solidified at 123'6. 



FREEZING MIXTURES. When certain substances are brought 

 into intimate contact, a more or less considerable degree of cold is 

 produced. Such mixtures are denominated freezing mixtures. 



The principle on which the action of freezing mixtures depends is 

 the following. When solids assume the liquid condition a large 

 amount of heat is always absorbed or rendered latent; and when 

 liquids assume the gaseous condition a large quantity of heat is also 

 thus takeu up. For example, when a piece of ice having a temperature 

 of 32 Fahr. is plunged into its own weight of water at 174 Fahr., 

 the ice immediately begins to liquefy, but during that liquefaction 

 enough heat is absorbed by it to bring down the temperature of the 

 hot water to that of the ice itself ; 142 degrees of heat having thus 

 disappeared, the thermometer taking no note of it. Again, the same 

 body, water, -when exposed in a vacuum under favourable circum- 

 stances, takes up so much heat in assuming the gaseous condition, that 

 portions of it are converted into ice ; the one part that boils away 

 robbing the remainder of the heat that kept it in the liquid state. 



From the fact that water in passing from the solid to the liquid 

 state has the property of taking up, or rendering latent, a greater 

 amount of heat than any other substance, it follows that solid water 

 must be a powerful agent in producing artificial cold, and this 

 if in practice found to be the case. It is employed in the form of 

 powdered ice or snow, or sometimes combined with certain salts, con- 

 stituting their so-called water of crystallisation. 



Before giving the formulae for some freezing mixtures, it will be as 

 well to premise that, an a general rule, the substances composing them 

 should be finely powdered, rapidly mixed, as large a quantity as wanted 

 made at one operation, and the whole placed in vessels possessing the 

 smallest possible amount of conducting power. The greatest amount 

 if cold obtainable with the substances used, is only produced when 

 they themselves, as well as the vessels used in the operation, are all 

 previously cooled by freezing mixtures. 



In addition to the freezing mixtures given under FKEEZINCJ, the 

 following may be mentioned aa useful frigorific agents : 



Mixtures. 

 Nitrat* of ammonia 

 Water . 



AllTS AM) SCI. HIV. VOL. IV. 



Thermometer 



sinks 



Parts. 

 .'."{ JFrom +50" to +4' 



Degree of cold 

 produced. 



= 46' 



Thermometer Degree of cold 



The rapid evaporation of ether in racuo has recently been rendered 

 available for the manufacture of ice upon a large scale, and as the 

 ether ia evaporated and condensed again by steam power, it may be 

 truly said of the process that it produces ice by the combustion of 

 coal. It is found that the combustion of one ton of coal yields about 

 five tons of ice. 



For common experimental purposes the most convenient freezing 

 mixtures are the sulphate of soda and hydrochloric acid, or salt and 

 powdered ice. 



FREIGHT. The charge made for the carriage of merchandise in a 

 ship, and the amount of which is generally specified in the bill of 

 lading. [BILL OF LADING.] It frequently happens that the whole 

 ship is hired by a merchant for the performance of the voyage, and in 

 this case a certain amount of freight is paid without reference to the 

 quantity of goods actually put on board, which may be sufficient to fill 

 the ship, or any quantity short of the same. In such cases the mode of 

 payment is part of the matter of agreement between the ship-owner 

 and the merchant, and the instrument by which this and other stipu- 

 lations are set forth is called a charter-party. Where no such instru- 

 ment exists, and the shipper of goods does not stipulate for the right 

 of using the entire portion of the ship appropriated to the reception of 

 her cargo, the amount of the freight, as well as the mode of payment, 

 is usually inserted in the bill of lading. Where this is not done, the 

 freight is by law considered due on the part of the merchant on the 

 delivery of the goods, and the owner or master of the ship may, if so 

 minded, demand payment of the same, package by package, as the same 

 are delivered. In almost all branches of trade, however, some custom in 

 this respect has arisen which is ordinarily pursued, and the legal rights 

 of the ship-owner are not enforced in this respect. In London, where 

 the greater part of the merchandise brought from foreign countries is 

 delivered into the custody of one or other of the incorporated dock 

 companies, a custom has arisen of arresting the goods in their hands, 

 so that they cannot pass away from the original importer until the 

 ship-owner, or some person acting on his behalf, has signified in writing 

 that the freight has been paid. If goods are damaged on board the 

 ship, through the carelessness or wilful neglect of those in whose charge 

 she and her cargo are placed, so that the owner of the ship is held to 

 be liable for the amount of the damage, this cannot, but with the con- 

 Bent of the owner or master, be set off against the amount of the 

 freight, which must under all circumstances be paid, and the merchant 

 must afterwards substantiate his claim to compensation for the amount 

 of the damage. 



FRENCH CHALK. [COLOURING MATTERS.] 



FRESCO PAINTING. A painting is said to be a fresco, or painted 

 in fresco (tul fresco intonaco, upon the fresh coat), when it is executed 

 in water-colours upon a freshly plastered wall, while the plaster is still 

 wet ; or upon wet plaster spread upon a wooden frame or any other 

 object. Fresco is the most noble and imposing of all methods of 

 painting. It does not admit of the softness, delicacy, or finish of 

 execution, or the richness and depth of colour, of oil-painting; but the 

 very want of the inferior beauties compels the painter to rely upon the 

 highest intellectual conception, composition, drawing, and expression, 

 united with a largeness of style and freedom of handling, which can 

 only result from great technical knowledge. It is therefore only in 

 the hands of the great painter that fresco is likely to be really successful. 



Colouring in fresco was practised by the ancients, though it has 

 not yet been shown that they painted frescoes. Some of the walls at 

 Pompeii are coloured in fresco, but their decorations are executed iu 

 some species of tempera upon the fresco coloured wall. This is evident, 

 because in many places the paintings have cracked off, and exposed the 

 original colouring of the ground in perfect freshness, and the same n 

 the rest of the wall where no decorations are painted. The Roman 

 term equivalent to fresco is udo tectorio, and it is used by Pliny (' Hist. 

 Nat.' xxxv. 31), who gives directions as to what colours it is safe or 

 unsafe to apply in this manner. Vitruvius (vii. 3) explains the mode 

 of preparing the walls for this species of colouring, and describes a 

 method of varnishing them when coloured, to preserve them. They 



