a problem. When they warm up, the water in the plants melts and can be disposed of, and things are simpler.

II

THE FLUORINE PLANET

1. THE STAR AND PLANET

The planet named Niflheim is the fourth planet of Nu Puppis, right angle 6:36, declension -43:09; B8 type star, blue-white and hot, 148 light years distant from Earth, which will require a speed in excess of light to reach it.

Niflheim is 462,000,000 miles from its primary, a little less than the distance of Jupiter from our sun. It thus does not receive too great a total amount of energy, but what it does receive is of high potential, a large fraction of it being in the ultra-violet and higher frequencies. (Watch out for really super-special sunburn, etc., on unwarned personnel.)

The gravity of Niflheim is approximately 1 g, the atmospheric pressure approximately 1 atmosphere, and the average ambient temperature about -60°C; -76°F.

2. ATMOSPHERE

The oxidizer in the atmosphere is free fluorine (F₂) in a rather low concentration, about 4 or 5 percent. With it appears a mad collection of gases. There are a few inert diluents, such as N₂ (nitrogen), argon, helium, neon, etc., but the major fraction consists of CF₄ (carbon tetrafluoride), BF₃ (boron trifluoride), SiF₄ (silicon tetrafluoride), PF₅ (phosphorous pentafluoride), SF₆ (sulphur hexafluoride) and probably others. In other words, the fluorides of all the non-metals that can form fluorides. The phosphorous pentafluoride rains out when the weather gets cold. There is also free oxygen, but no chlorine. That would be liquid except in very hot weather. It sometimes appears combined with fluorine in chlorine trifluoride. The atmosphere has a slight yellowish tinge.

3. SOIL AND GEOLOGY

Above the metallic core of the planet, the lithosphere consists exclusively of fluorides of the metals. There are no oxides, sulfides, silicates or chlorides. There are small deposits of such things as bromine trifluoride, but these have no great importance. Since fluorides are weak mechanically, the terrain is flattish. Nothing tough like granite to build mountains out of. Since the fluoride ion is colorless, the color of the soil depends upon the predominant metal in the region. As most of the light metals also have colorless ions, the colored rocks are rather rare.

4. THE WATERS UNDER THE EARTH

They consist of liquid hydrofluoric acid (HF). It melts at -83°C and boils at 19.4°C. In it are dissolved varying quantities of metallic and non-metallic fluorides, such as boron trifluoride, sodium fluoride, etc. When the oceans and lakes freeze, they do so from the bottom up, so there is no layer of ice over free liquid.

5. PLANTS AND PLANT METABOLISM

The plants function by photosynthesis, taking HF as water from the soil, and carbon tetrafluoride as the equivalent of carbon dioxide from the air to produce chain compounds, such as:

and at the same time liberating free fluorine. This reaction could only take place on a planet receiving lots of ultra-violet because so much energy is needed to break up carbon tetrafluoride and hydrofluoric acid. The plant catalyst (doubling for the magnesium in chlorophyll) is nickel. The plants are colored in various ways. They get their metals from the soil.

6. ANIMALS AND ANIMAL METABOLISM

Animals depend upon two main reactions for their energy, and for the construction of their harder tissues. The soft tissues are about the same as the plant molecules, but the hard tissues are produced by the reaction:

resulting in a teflon boned and shelled organism. He's going to be tough to do much with. Diatoms leave strata of powdered teflon. The main energy reaction is:

The blood catalyst metal is titanium, which results in colorless arterial blood and violet veinous, as the titanium flips back and forth between tri and tetra-valent states.

7. EFFECT ON INTRUDING ITEMS

Water decomposes into oxygen and hydrofluoric acid. All organic matter (earth type) converts into oxygen, carbon tetrafluoride, hydrofluoric acid, etc., with more or less speed. A rubber gas mask lasts about an hour. Glass first frosts and then disappears. Plastics act like rubber, only a little slower. The heavy metals, iron, nickel, copper, monel, etc., stand up well, forming an insoluble coat of fluorides at first and then doing nothing else.

8. WHY GO THERE?

Large natural crystals of fluorides, such as calcium difluoride, titanium tetrafluoride, zirconium tetrafluoride, are extremely useful in optical instruments of various forms. Uranium appears as uranium hexafluoride, all ready for the diffusion process. Compounds of such non-metals as boron are obtainable from the atmosphere in high purity with very little trouble. All metallurgy must be electrical. There are considerable deposits of beryllium, and they occur in high concentration in its ores.

PROLOGUE

On Satan's Footstool

The big armor-tender vibrated, gently and not unpleasantly, as the contragravity field alternated on and off, occasionally varying its normal rate of five hundred to the second when some thermal updraft lifted the vehicle and the automatic radar-altimeter control acted to alter the frequency and lower it again. Sometimes it rocked slightly, like a boat on the water, and, in the big screen which served in lieu of a window at the front of the control cabin, the dingy-yellow landscape would seem to tilt a little. If unshielded human eyes could have endured the rays of Nu Puppis, Niflheim's primary, the whole scene would have appeared a vivid Saint Patrick's Day green, the effect of the blue-predominant light on the yellow atmosphere. The outside 'visor-pickup, however, was fitted with filters which blocked out the gamma-rays and X-rays and most of the ultra-violet-rays, and added the longer light-waves of red and orange which were absent, so that things looked much as they would have under the light of a G0-type star like Sol. The air was faintly yellow, the sky was yellow with a greenish cast, and the clouds were green-gray.

A thousand feet below, the local equivalent of a forest grew, the trees, topped with huge ragged leaves, looking like hundred-foot stalks of celery. There would be animal life down there, too—little round things, four inches across, like eight-legged crabs, gnawing at the vegetation, and bigger things, two feet long, with articulated shell-armor and sixteen legs, which fed on the smaller herbivores. Beyond, in the middleground, was open grassland, if one could so call a mat of wormlike colorless or pastel-tinted sprouts, and a river meandered through it. On the skyline, fifty miles away, was a range of low dunes and hills, none more than a thousand feet high.

No human had ever set foot on the surface, or