parts.  Once  a  small  amount  of  sludge  or  other deposits  is  formed,  the  rate  of  formation  generally increases  more  rapidly.  As  these  deposits  are formed,  certain  changes  in  the  physical  and chemical properties of the liquid take place. The liquid  usually  becomes  darker,  the  viscosity increases  and  damaging  acids  are  formed. The  extent  to  which  changes  occur  in  different liquids  depends  on  the  type  of  liquid,  type  of refining,  and  whether  it  has  been  treated  to provide  further  resistance  to  oxidation.  The stability  of  liquids  can  be  improved  by  the addition  of  oxidation  inhibitors.  Inhibitors selected to improve stability must be compatible with the other required properties of the liquid. FREEDOM  FROM  ACIDITY An ideal hydraulic liquid should be free from acids which cause corrosion of the metals in the system.  Most  liquids  cannot  be  expected  to  remain completely  noncorrosive  under  severe  operating conditions. The degree of acidity of a liquid, when new, may be satisfactory; but after use, the liquid may  tend  to  become  corrosive  as  it  begins  to deteriorate. Many systems are idle for long periods after operating  at  high  temperatures.  This  permits moisture to condense in the system, resulting in rust  formation. Certain  corrosion-  and  rust-preventive  addi- tives are added to hydraulic liquids. Some of these additives are effective only for a limited period. Therefore, the best procedure is to use the liquid specified for the system for the time specified by the  system  manufacturer  and  to  protect  the  liquid and  the  system  as  much  as  possible  from contamination by foreign matter, from abnormal temperatures,  and  from  misuse. FLASHPOINT Flashpoint is the temperature at which a liquid gives  off  vapor  in  sufficient  quantity  to  ignite momentarily or flash when a flame is applied. A high flashpoint is desirable for hydraulic liquids because  it  provides  good  resistance  to  combustion and  a  low  degree  of  evaporation  at  normal temperatures.   Required   flashpoint   minimums vary from 300°F for the lightest oils to 510°F for the heaviest oils. FIRE POINT Fire  point  is  the  temperature  at  which  a substance  gives  off  vapor  in  sufficient  quantity to  ignite  and  continue  to  burn  when  exposed  to a spark or flame. Like flashpoint, a high fire point is  required  of  desirable  hydraulic  liquids. MINIMUM   TOXICITY Toxicity  is  defined  as  the  quality,  state,  or degree of being toxic or poisonous. Some liquids contain chemicals that are a serious toxic hazard. These toxic or poisonous chemicals may enter the body  through  inhalation,  by  absorption  through the skin, or through the eyes or the mouth. The result  is  sickness  and,  in  some  cases,  death. Manufacturers  of  hydraulic  liquids  strive  to produce  suitable  liquids  that  contain  no  toxic chemicals and, as a result, most hydraulic liquids are  free  of  harmful  chemicals.  Some  fire-resistant liquids are toxic, and suitable protection and care in  handling  must  be  provided. DENSITY  AND  COMPRESSIBILITY A fluid with a specific gravity of less than 1.0 is desired when weight is critical, although with proper  system  design,  a  fluid  with  a  specific gravity greater than one can be tolerated. Where avoidance  of  detection  by  military  units  is  desired, a fluid which sinks rather than rises to the surface of the water is desirable. Fluids having a specific gravity  greater  than  1.0  are  desired,  as  leaking fluid will sink, allowing the vessel with the leak to  remain  undetected. Recall  from  chapter  2  that  under  extreme pressure  a  fluid  may  be  compressed  up  to  7 percent  of  its  original  volume.  Highly  com- pressible fluids produce sluggish system operation. This does not present a serious problem in small, low-speed operations, but it must be considered in the operating instructions. FOAMING  TENDENCIES Foam  is  an  emulsion  of  gas  bubbles  in  the fluid.  Foam  in  a  hydraulic  system  results  from compressed gases in the hydraulic fluid. A fluid under high pressure can contain a large volume of air bubbles. When this fluid is depressurized, as when it reaches the reservoir, the gas bubbles in  the  fluid  expand  and  produce  foam.  Any amount  of  foaming  may  cause  pump  cavitation and  produce  poor  system  response  and  spongy 3-4