Explosives are grouped into two main classes, low explosives, which burn at rates of inches per second, and high explosives, which undergo explosion at rates of from 914 to 9140 m per sec (1000 to 10,000 yd per sec). Explosives vary in other important characteristics that influence their use in specific applications. Among these characteristics are the ease with which they can be exploded and their stability to conditions of heat, cold, and humidity. The breaking effect, or brisance, of an explosive depends upon the velocity of explosion. Some of the newer high explosives with a exolsion rate of 9140 m per sec are extremely effective for military destruction and certain types of blasting. On the other hand, for quarrying and mining, when it is desirable to dislodge large pieces of rock or ore, explosives with a lower explosion velocity and lower brisance must be employed. Explosives used as propellants in rifles and cannon should burn still more slowly, as they are required to deliver a steadily increasing push to the projectile in the barrel of the gun rather than a sudden shock which, if strong enough, might break the gun. Special types of explosives that are sensitive to heat or shock and have a medium-high brisance are used to initiate the explosion of less sensitive high explosives. High explosives are often mixed with inert materials to reduce sensitivity and lower brisance, as in the case of dynamite.
A great number of explosives undergo explosion. Some of these, such as TNT, trinitrotoluene, have a high resistance to shock or friction and can be handled, stored, and used with comparative safety. Others, such as nitroglycerin, are so sensitive that they are almost invariably mixed with an inert desensitizer for practical use. To obtain desirable characteristics, explosives of different characteristics are often mixed. During World War I, TNT was the high explosive most generally employed, but before and during World War II a number of extremely efficient new high explosives were developed. Among the most important are cyclonite and pentaerythritol tetranitrate. Cyclonite, also called RDX, is used in detonators. A mixture with TNT and wax is called Composition B and is used in bombs. A similar mixture, containing aluminum and called torpex, has an underwater effect about 50 percent greater than that of TNT. A plastic composition containing cyclonite and an explosive plasticizer is used for destructive charges. Pentaerythritol tetranitrate, also called PETN, has characteristics similar to those of cyclonite and is mixed with TNT to form the explosive pentolite. It also forms the core of the explosive primacord fuses used for exploding destructive charges and the booster charges used in blasting. Two types of high explosives introduced since 1955 have largely replaced dynamite. A mixture of ammonium nitrate and fuel oil has explosive strength 25 percent greater than that of TNT. The so-called slurry explosives, which are also used for blasting, are mixtures containing sufficient water to form flowable material.
In coal mining the use of ordinary high explosives is hazardous because of the danger of igniting gases or suspended coal dust that may be present underground. For blasting under such conditions several special types of safety explosives have been developed that minimize the danger of fires or explosions by producing flames that last for a very short time and are relatively cool. The types of safety explosives approved for work in coal mines are chiefly mixtures of ammonium nitrate with other ingredients such as sodium nitrate, nitroglycerin, nitrocellulose, nitrostarch, carbonaceous material, sodium chloride, and calcium carbonate. Another kind of blasting charge for use in mining has grown in favour, because it produces no flame whatsoever. This charge is a cylinder of liquid carbon-dioxide that can be converted into gas almost instantaneously by an internal chemical heating element. One end of the cylinder contains a breakable seal through which the gas can expand. The carbon dioxide charge is not a true explosive and absorbs heat rather than evolving it. It has the additional advantage that the force of the explosion can be directed at the base of the bore hole in which the charge is placed, thus lessening the breaking of the coal.
It is hard to imagine a more dramatic scene than the predawn hours at Trinity Site on July 16. If the gadget exploded in the relatively low range of 3,000 tons of TNT, it would be only slightly more powerful than a standard “blockbuster” weapon, hardly worth the huge expense and effort. If the bomb were a “dud,” the Manhattan Project would rank as the most costly industrial failure of all time. On the other hand, some scientists formed an opinion that if the blast exceeded expectations, it could conceivably ignite Earth’s atmosphere and end all life as we know it. At the local time of 5:29:45 am, July 16, 1945, the atomic age began at Trinity Site. The blast vaporized the steel tower, tore huge chunks out of the earth, and broken windows 200 km (125 mi) away. A large multicolored mushroom cloud rose to an altitude of 12,000 m (40,000 ft) within minutes and began to move slowly to the northeast. Where the ball of fire touched the earth, it fused the sand into a radioactive greenish-gray glass—later named Trinitite—that resembled “a sea of green.” All living things within the radius of a kilometer—birds, plants, snakes, lizards, rodents—were instantly incinerated.Wild antelope ran terrified in all directions. The yield was estimated at around 20,000 tons of TNT.Since the blast was visible in three states—indeed, it could have been seen from the Moon—one might expect that news about it would dominate the headlines. But U.S. Army security had instructed all regional newspapers to print only the “official” version of the events, which was that an ammunition dump had exploded accidentally. At first the scientists who observed the blast were overjoyed. They had cracked the secret of atomic weapons. Germany had surrendered on May 7, 1945, and the war against Japan would soon be over. No nation could withstand the power of such a weapon. But within moments, several scientists began to have second thoughts. Fermi became temporarily ill from the stress and worry. Oppenheimer at first remarked that his confidence in the human mind had been restored, but later, quoting from the epic Hindu poem, the Bhagavad-Gita, he solemnly observed, “Now I am become Death, the destroyer of worlds.” James Tuck of the British Mission summed up the thoughts of many who watched the cloud roil the summer sky: “What have we done?” The news of the success at Trinity was immediately cabled to officials in Washington, D.C., who sent it on to President Harry S. Truman. Truman was in Germany meeting with Soviet and British leaders at the Potsdam Conference to discuss the best way to end the war with Japan. Truman read the report with delight, but he did not immediately inform the Soviets, who had been excluded from the Manhattan Project. Instead, he later casually mentioned to Soviet premier Joseph Stalin that the United States had developed a powerful new weapon. Stalin, just as casually, according to Truman’s memoirs, said he hoped it would be put to good use against the Japanese. Although Soviet spies had gathered some information on the Manhattan Project, scholars are still debating how much Stalin actually knew when Truman first told him about the Trinity Site explosion.