Year: 1877



 Diffusion of Heat.-Heat passes, or is ditfused, or is conveyed from one place or point to another in three different ways, termed cORductiOn, correction, and radiation.

 CONDUCTION.

 If one end of an iron rod be placed in the flame of a lamp. or in a fire, the op- posite end will soon become warm, even though the rod be quite long and the opposite end at 8 considerable distance from the fire. The heat generated in the fire by the combustion of the fuel has been conducted along the rod to it's furthest extremity. As may be seen from this, the passage of heat from one body to another depends upon law which may be expressed as follows: U one or tlUO particlee of matter, situated lC6U giUe up its beat to the other until both become of the some tenperature.

 If we place hand on piece of wood in a cold room, the sensation will be very different from that perceived when marble or iron IS touched. The wood or piece of cloth may seem warm, while the marble or iron may feel cold. From this it would seem that the law expressed above has eX- captions, but in reality such is not the case, for if thermometer be applied to the various objects. the temperature will be found to be the same in all. The reason that the marble and iron feel quite cold, is that they conduct heat quite readily, so that the tempera- ture of the hand is at once reduced, and the sensation of cold is produced. The cloth and wood, being poor con- ductOrs of heat, do not absorb the heat from the hand, and consequently feel warm.

 CONVECTION,

 is the motion of the hot body itself carrying it's heat with it and giving it up to other bodies as it comes in con- tact with them. In boiling water, the hot particles near the source of the heat in the bottom of the vessel, be- coming lighter as they grow warm, rise to the surface of the liquid, warming the othe molecules of it, It is in this way that the air about the stove be- comes heated. As the air about the stove becomes hot, it rises to the upper part of the room, heating the more remote and colder particles as i. rises, until it has parted with its heat, and it's temperature has been reduced to that of particles of air surrounding it. The same is true of other liquids and gases.

 RADIATION.

 The third method of diffusion of heat is that in which the heat passing from one substance to another, does not warm the medium through which it passes. On calm day in winter, the suns rays will warm the body, even though water freeze and ice remain dry. If we place a thermometer in tho path of the ray of heat from the sun, it will indicate 8 temperature far above freezing. Heat passing into water this way does not warm the water, unless it contains particles of solid matter of considerable size. The particles of solid matter interfere with the passage of the rays-stop it, and they, acting under the law given above, impart their heat to the water. Substances that allow radiant heat to pass through them are termed diathermanous, Sub stances that do not allow heat to pass through them without becoming warm are termed athermanOus.

 HoaU to i,ake Printing Plates by Pho tography.-M. Boivin, who is persever- ingly pursuing his labors and interest ing researches, has written a descrip- tion of very facile process to obtain

  printed on an ordinary printing or en- graving press. Unfortunately it is impossible, so far. to reproduce half tones by these means; but neverthe less, the process will be valuable for reproducing linear designs sketches. When it is desired to produce block or printing plate in relief, 8 sheet of zinc or copper is taken, or 1-5 inch in thickness. After having gained the surface it is coated, in a warm condi- tion. with light film of wax. To this film you transfer 8 car bon print by ordinary means, and having developed it, it is dipped in alum solution and dried. Then the plate is plunged into some solvent of wax-benzole, for in- stance-and in this way those portions of the metal surface not covered by the image are laid bare, ready to be etched with acid; the layers of wax and car bon in the other parts are sufficient protection against any mOrdant that may be used, It need scarcely be said that, when blocks for the printing press are desired, a negative cliche must be made use of; while in that produced off press, positive image must be em ployed. Nevertheless, an ordinary negative may also be employed in the the latter case; only, if this is done after having produced the carbon pie ture, the plate must be covered with a film of copper by the electrotype pro- cess. The image may be removed by hypochlorite of lime and boiling water, and then the wax with benzole. and finally acid is employed to etch. In the latter case the the bare zinc plate is etched by the acid. The success of this process of photo-engraving is assured, according to M. Boivin, if use is made of very clear negatives, presenting opaque blacks and whites free from fog. It is indispensable, also, that the pigmented tissue has never been exposed to light previously. M. Boivin finds that the employment of wax is more facile than that of asphalt dissolved in the the benzole,' recommended by M. Markl for an analogous process.-E. Lacan, in Photographic NeaUs.

 Spirit Lace. - We are greatly in debted to a correspondent for 8 speci- men of lace which, he informs us, was "materialized" at 8 recent spir4tualistic seance. We have long desired to ex- anime some of these marvellous pro- ductiOns of the elect, hoping thereby to acquire knowledge of some of the phenomena not "known to our philos- ophy." It is with the greater regret, therefore, that we perceive that the spirits have not improved upon the goods manufactured by base mortals, and that the tlimsy cloud wreaths, out of which this veil was undoubtedly formed, are capable of no better ma terialization than as a cheap quality of tulle, stiffened with unspiritual British gum. The only thing about the fabric which savers of the spiritualistic is that it is of the variety of lace com