ANCIENT philosophers divided the material world into four great elements, namely : fire, air, earth and water, and they supposed that all bodies were constituted of these. Modern chemists have already discovered more than sixty different elements ; yet, in a general sense, ancient science was not so very far wrong. At present, we do not know whether heait is a subtle ether or simply an action of m a 11 e r, but we know a great deal a bo u t Its op e ra t i o ns and effec t s. All organic c re at i o n is dependent upon the proper distribution of heat, moi s t-uro, ail', and earth (food). Deprive mnn of any one of these elements for a c er ta i n length of time, an d he will cease to live. And as the normal temperature of h is blood is the same in every climate, if it be elevated above 100^ Fah., or depressed much below it, he soon b ec o mes depressed and h el pl e s s. Man is so constituted that he only requires food and witer at intervals of sev-eral hours, but it is fcU* otherwise with air. A constant s uppl y of t h i s clement is n e c ess ary every moment of his e x i s t e n c e ; he takes in fre s h air ami expels c arb on i c acid gas at every re s pir a tion. Our b od i es must be m ai n-tained at the stime heat in all plaices and at all periods of th e year, and yet we live in a cl imate the temperature of which mnges fi'om blood hcat to more than a hund red deg rees below it. The air w h i c h we b re a th e is the great vehicle of ch anges in ou r atmosphere. 11 comes wirm from the soiith, mild f!"Om the west, and piercing cold from the icy north. To maintain health and life, th ere fo re, we req ui re the element air in certain q uan t i ti e s, and gene r al ly in a wtirm c on d i ti on. A few w or ds at this season o f the ye a r, o n t ll i i i m p ortan t subject, may b e of great service t o m a ny p erso ns. In nature, the cuITents of the atmosphere distribute pure a i r OI'er t he entire surface of the gl o b s. The pri -mary source of these cmrents is heat. It rarefies one stratum of aii', causing it to ex t end, th u s le a v i ng a pair-tial vacuum which is instantly filled np by the frce cool air which forms an under clurent, thus maintaining a constant circuhati o n. T h e t r ne th eory, t h e re fo re, is the pro duc t ion of a c urre n t of aii* by the dis plac em e n t of the impure with a supply of fi'esh air. So efficient is this natural system of v e n til a t ion that c he m i s ts have been unable to d e te et any difference between the f^ir of the most crowded cities and that of the m os t t h i n ly peo pl e d hamlets. It would be well for humanity if this system was m ore th oro u ^hly understood and cnnied out, in cold climates, by those who occupy dwel11ngs. 'Yhen, on a c ol d day, we enter m os t p ubl i c buildings, churches, w ork s h o ps, s t o re s a n d d w e 11 i n 1): hou s e s, we become sensible of the presence of no x i o u s g as es, inde pe n dent of the warmth of the inclos ed a t mos ph e -e. The se gi s e s are ge n e ral 1 y exhalation.s from tI.e l u n g s ; th ey tend to diminish v i tali ty and p ro d uce disease. In cold w e at h e r, it is positively necess ary to h eat the at m os ph e re of rooms, in o rd e r to m ai n t ai n the body .at a c on s t a n t temperature, but p ro v i s i on should a l w ays be mi de for the production of an artificial atmospheric current. In apar t Gl en t s heated by fi re s in open grates, t his system is gener al ly carried out in the m o st si m pie manner, and in b u i ld i n gs heated by s team pipes, or hot air furnaces, when s u i table ven t i lato rs o pe ning outward and placed n e ar t he c e il in g, a re e m p 1 oye d, the same results are attain ed, bec au se n n d e r curren ts o f c old ai r gen eral ly find access under the doors and by o ther chinks in th e rooms. But th ere are thousands upon thousands of houses heated by s t ov e s and fu r n ac es in which n o p ro vision is m a de for the exi t of the i m pure ai r, a n d, co nsequ e n tly, no artificial current is formed in them. Fevers are very common i n s uch d wel lings A m ist ake is frequently made resp ec ti ng the p uri ty of warm and c old air in h o u s es. A cold room may contain very i mpure ai r, because it lI1Jly have remained un -chnnged for several days, just for want of a little fire to pro d iice an atm osph er ic current, while on the other hand, a warm apartment may contain very pure air, o w i n g to the m a i nte n an c e of a cons ta n t current in it. With a d i s ti n c t under.standing of these views, every house may be v e n t i la te d in the m o st e ffi c ient and s i m pl e manner by the very agents which we re qu i re to heat the m ; this is na tu re's plan. There can be no doubt of the sal u b r i ty of w a r m houses in w i n te r, and it see m s that th^colder the climate, so in the same degree the h u m an fra m e r equi res w ar m e r d w el l i ngs. The great o bj ec t to which attention should be pai da nd there can be no excuse for neglccting itis to secure a con-stnnt and sufficient s u p ply of w arm fresh air. In th is bi t of knowledge is concentrated the true theory of artificial heating and ventilation.