PHO 



PHO 



plants. This is the thalamus and cli- 

 nanthium of other writers. 



PHOSGENE GAS (0(Sf, light, ^ei/i/dto, 

 to produce). Chloro-carbonic acid gas; 

 a compound of chlorine and protoxide of 

 charcoal. It is named from the peculiar 

 power of the sunbeam in effecting this 

 combination. 



PHOSPHATE. A salt formed by the 

 union of phosphoric acid with a sala- 

 fiable base. 



PHO'SPHITE. A salt formed by 

 the union of phosphorous acid with a 



PHOSPHORE'SCENCE (05?, light, 

 0epa), to carry). The emission of light 

 by many bodies at a low temperature, 

 unaccompanied by any essential change 

 in their properties. The luminous ap- 

 pearance of common phosphorus is the 

 result of chemical action, for it takes 

 place only when this substance is in 

 combination with oxygen or chlorine. 



1. Phosphorescence by insulation is a 

 property possessed by many bodies, in 

 consequence of which, after long ex- 

 posure to the sun's rays, they become 

 luminous in the dark on the application 

 of heat. Of this kind are the phosphorus 

 of Baldwin, or the ignited muriate of 

 lime ; phosphorus of Canton, consisting 

 of oyster-shells, calcined with sulphur; 

 and the Bologna stone, or the sulphate of 

 barytes. 



2. Phosphorescence of organic remains 

 in a state of dissolution. This luminous 

 property occurs in touchwood, and in 

 sea-fish, on which a luminous film is 

 formed. The phosphorescence appears 

 to be owing to a low sort of combustion, 

 as the emission of light decreases on 

 rarefying the air. 



3. Phosphorescence of organic bodies 

 during life. This occurs in the marigold 

 and yellow lily, a little after sunset, when 

 the atmosphere has been very much 

 heated in July and August. It is pro- 

 bably of an electrical nature. The lu- 

 minosity of the infusoria, zoophytes, 

 fire-flies, and glow-worms, is a subject of 

 much obscurity. 



PHO'SPHORITE. A sub-species of 

 apatite, comprising the fibrous and the 

 compact phosphates of lime. 



PHO'SPHORUS ((pw, light, ^epco, to 

 carry). A solid, semi-transparent colour- 

 less substance, of a waxy consistence, 

 undergoing slow combustion at ordinary 

 temperatures ; burning brilliantly in air. 

 Combined with lime, it is a principal 

 constituent of the bones of verlebrated 

 258 



animals, and is found in some kinds of 

 limestone. 



1. Phosphorous acid. An acid pro- 

 duced, in the form of a white volatile 

 powder, by the slow combustion of 

 phosphorus. Its salts are called phos- 

 phites. 



2. Phosphoric acid. An acid obtained, 

 in the form of white flakes, by igniting 

 phosphorus under a large bell jar. Its 

 salts are called phosphates. 



PHO'SPHURET. A compound of 

 phosphorus with a combustible or a 

 metallic oxide. 



PHOSPHY'TTRITE. Phosphate of 

 yttria ; a very scarce mineral substance, 

 first found in the granite of Lindenas in 

 Norway, and subsequently in equally 

 small quantities at Ytterby in Sweden. 



PHO'TICITE. A mixture of the sili- 

 cate and the carbo-silicate of manganese. 



PHOTOGE'NIC DRAWING (05)r, 

 ^toTor, light, 7ev»/a'&), to produce). A 

 modern discovery by which objects are 

 represented by the chemical action of 

 light on a prepared metallic tablet, upon 

 which the images of the objects are 

 thrown by a camera obscura. The dis- 

 covery was made by Daguerre ; and the 

 apparatus was hence called Daguerreo- 

 type ; and the process itself photogeny, 

 photography, or heliography. 



PHOTO'GRAPHY (^Sf, ^wtoc, light, 

 fpd<p(a, to delineate). Heliography. The 

 application of the chemical effects of 

 light to the purpose of rendering perma- 

 nent the images obtained by means of 

 convex lenses. See Daguerreotype. 



PHOTO'METER (0Sf, <po)r6i, light, 

 fxerpov, a measure). An instrument lor 

 measuring the relative illuminating 

 powers of different sources of light. In 

 Rumford's photometer, the depth of the 

 shadow is the measure used for com- 

 paring the intensities of different lights ; 

 in Ritchie's, the brightness of the illu- 

 mination answers the same purpose. 

 When the shadows are equally deep, or 

 when the illuminations are equally 

 bright, the luminous powers of the two 

 sources of light will be as the squares of 

 the distances at which they act. 



Leslie's Photometer consists of the 

 differential thermometer with one of the 

 balls blackened. The clear ball transmits 

 all the light that falls upon it, and there- 

 fore its temperature is not affected ; the 

 black ball, on the contrary, absorbs all 

 the light, and a corresponding elevation 

 of temperature takes place. The action of 

 the photometer depends, therefore, on 



